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Collecting retrospective data: Development of a reliable method and a pilot study of its use
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Soc. Sci, Med. Vol. 42, No. 5, pp. 751-757, 1996
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D. B. BLANE Academic Department of Psychiatry, Charing Cross and Westminster Medical School. London W6 8RP, England Abstract--The present paper argues that a need will remain for data which have been collected retrospectively. Recent developments in oral history and sociology are described to suggest a method of collecting retrospective data which may minimize recall bias. Pilot work is reported on the adaptation of the method for a study of chronic respiratory disease. The results of the pilot study proved promising. The technical properties of the measures appeared adequate. The results of the substantive analysis were consistent with existing knowledge and went beyond existing knowledge to suggest new areas of research. Ways of further validating the method are identified and its wider application discussed.
Key words--retrospective data, life grid, lifetime exposure, chronic respiratory disease
INTRODUCTION The present paper reports the early stages of a program of work designed to develop a method of studying the development and causes of chronic disease. The pilot work has been concerned with one chronic disease, namely, chronic obstructive airway disease (COAD), which unites the previously distinct diagnoses of emphysema and chronic bronchitis. In common with the 'new epidemiology', the work draws on the ideas of both the natural and social sciences and integrates qualitative and quantitative methods. It is intended to help answer patients' questions about the causes of their disease, an area where at present medicine too often has to confess its ignorance. Those studying the origins and development of chronic diseases usually share the view that retrospective data are unreliable and subject to significant recall bias [1]. Recall bias is seen as particularly hazardous to the study of chronic disease, because these diseases often develop slowly over several decades. There is adequate time for the subjective re-working and re-interpretation of past events as individuals attempt to explain the calamity that has befallen them. Longitudinal studies eliminate most of the problems of recall bias and have become the preferred method of study in this area. At present there are at least 20 such studies under way in Britain 12]. Before describing the pilot work and its results, the paper examines the limitations of longitudinal studies and suggests that a need remains for data which have been collected retrospectively. The longitudinal and retrospective approaches are not seen as competitors. ssr~42/s---t
Both approaches are essential and they complement one another. The present paper is concerned with the retrospective approach and identifies recent developments which may minimize the unreliability of data collected in this way.
LIMITATIONS OF LONGITUDINALSTUDIES In relation to chronic disease, practical issues limit the usefulness of longitudinal studies. The most powerful longitudinal study tracks a birth cohort from birth through life. The earliest birth cohorts in Britain were born in 1946 [3] and 1958 [4] and the members of these cohorts are presently aged in their mid-40s and mid-30s respectively. At these ages most of the prevalent chronic diseases are in the early stages of their natural history and manifest disease is unusual. Chronic diseases develop over many decades, but in most cases they do not become manifest as overt pathology until late middle age or older. By the time the chronic diseases in these cohort members reach the clinically significant part of their natural history, the present generation of researchers will have long retired. Because birth cohorts take a long time to yield results in this area, the need remains for methods of study which can produce answers more quickly. Some studies attempt to avoid this problem by starting at later age groups where chronic diseases can be expected to become common during the first 10 or 20 years of follow-up [5, 6]. Such studies necessarily remain blind to events in the earlier part of life or they have to use retrospective methods to obtain this information. 751
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Sample attrition is a second limitation of longitudinal studies. Careful management can reduce the attrition rate to remarkably low levels, but longitudinal studies nevertheless lose a proportion of their sample, and this proportion tends to increase over time. Losses occur for a number of reasons, among the most important of which are refusal to further participate in the study and high levels of geographic mobility which make subjects difficult to trace. Losses to the original sample are likely to be socially selected and Granada Television's Seven-Up, which can be seen as a TV longitudinal study, suggests the nature of this selection. By the fourth sweep, at age 28, three of the original 15 subjects had dropped-out of the study. The two who refused to participate further were members of the service class, a barrister and a BBC executive. The third was geographically mobile, and so hard to trace, was unemployed and in poor health. Data from academic studies confirm this impression. Losses to the National Survey of Health and Development at age 15, for example, came disproportionately from the extremes of the social hierarchy, on the one hand subjects whose parents were owner-occupiers and possessed high educational qualifications, on the other hand those living in overcrowded housing. By age 36, those with low educational qualifications and those suffering from a psychiatric illness had also been disproportionately lost to the study [3]. Attrition to the National Child Development Study reduced the sample to 79.5% by age 16, with those from disadvantaged groups being most likely to be lost to follow-up [7]. This problem of selective attrition is not unique to longitudinal studies; it equally affects cross-sectional studies, where it is caused by similar factors. Third, longitudinal studies are vulnerable to the emergence of new research questions. The data which are collected in any particular sweep are relevant to the research questions of that decade. No study can predict which information about, say, 15 year olds will be relevant to the research questions that are current 30 years later. The missing data may be of one type, such as a particular measure of health, or cover a whole field of enquiry, such as the quality of material life. If current research interests requires such missing data, then their collection will probably depend on the retrospective method. Similarly, because results from longitudinal studies are not quickly available, by the time earlier conditions are evaluated they may be less relevant to current circumstances due to changes occurring in the intervening period. Practicality, doubts about representativeness and the emergence of new research questions all suggest a continuing need for data which have been collected retrospectively. To be useful, the method of their collection needs to minimize the well-documented sources of their unreliability [l]. Recent developments in oral history and sociology suggest two ways in which this might be achieved.
CHARACTERISTICS OF ACCURATE RECALL
Awareness of the unreliability of retrospective data has diverted attention from the potential accuracy of longterm memory. Studies which compare recalled basic occupational information against official records, for example, typically find around 80% agreement between these two sources [8-10]. A comparison of maternity records and mothers' accounts of the same events found similarly high levels of agreement on many items [11]. A study which compared recalled information obtained by contemporary interviews with medical and other records from 30 years earlier, also found high levels of agreement between the recorded and the recalled versions; on the items concerning breast feeding, for example, there were no discrepancies at all in 19 out of the 21 cases [12]. The studies cited above required specific information about discrete events. If equally reliable data about the more general aspects of life are to be obtained, it would be useful to know which types of event are remembered most accurately. Tests of recall after several months suggest that its accuracy is influenced by the emotional load attached to a particular item of information. Emotionally laden events are least likely to be recalled acccurately; in an extreme example, neonatal resuscitation was recalled less accurately than whether the child had been breastfed [11}. The emotional load carried by an event may also be important to memory over the longer periods of time relevant to the natural history of chronic disease [13]. Volunteer civilians recorded the details of life for Mass Observation during World War I! [14]. These records often differ from present day accounts of the same events by the same individuals, particularly where the events being recalled were traumatic, such as a bombing raid [! 5]. Conversely, hum-drum events which carry little emotional charge and the barely noticed background routines of life appear to be recalled most accurately [16]. Attempts to collect complex data retrospectively may benefit from avoiding items of information which are emotionally loaded and could usefully concentrate on the background, hum-drum details of life. The comparative accuracy with which emotionally neutral items are recalled has an interesting implication for the study of chronic respiratory disease. Public health education and the advice of doctors during consultations attach overwhelming importance to cigarette smoking as the cause of respiratory disease and pay comparatively little attention to its other known causes. The subjects of the presently reported study, for example, were often puzzled by questions about factors other than cigarette smoking. If events which are publicly accorded little importance tend to be recalled more accurately than those which are singled out for public attention, then items about occupational fumes and dusts, atmospheric
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Table I. Variables and their indicators Variable
Type of measure
Indicators
Residential damp
Indirect via population density Direct
Physically arduous work
Direct
Occupational fumes and dusts
Direct
Inadequate nutrition
Indirect. via household structure and income Direct
Continuously built-up area with most buildings at least three stories above ground level Wallpaper peels off walls; mould grows on internal walls; clothes steam when aired after storage Work perceived as heavy: heavy sweating; back injuries frequent among workforce Nature of work process; site (e.g. basement); level of ventilation (e.g. out-of-doors, extractor fans) One social class IV or V income coming into a household containing more than two children Smoking five or more cigarettes per day
Air pollution
Cigarette smoking
pollution and residential damp may be recalled more accurately by those suffering from chronic respiratory disease than the details of their cigarette smoking. The Social Change and Economic Life study has contributed a further advance in methods by developing the Life Grid [17, 18]. A full description of this method has yet to be published, but it was described and much discussed during the BSA annual conference on sociology and history in 1988. The method involves cross-referencing the dates of any changes in the areas of interest, for example occupation and housing, against dates in the subject's personal life, such as marriage and death of mother, as well as against events in the external world, like coronations and wars. Further details and examples are contained in the Appendix. Cross-referencing on the life grid enables subjects to improve the accuracy with which dates are remembered, which allows better estimates of the duration of events to be calculated. The life grid also appears to release detail from memory by juxtaposing different information from the same period of life. One subject, for example, was surprised to be able to list the chemicals he had used when employed as a chromium plater before World War II (nickel, chromium, copper sulphate, cyanide, sulphuric acid and caustic soda), something he claimed not to have thought about for 40 years. The data collected by the present study could have been analysed using event history techniques [18-22]. Event history analysis is concerned with changes in status, such as that from employed to unemployed, and calculates the probability that a specific change will be associated with a given outcome, such as increased mortality. A simpler method of analysis could be used in the present pilot study because a small number of subjects were involved and because previous research on respiratory disease pointed to a small number of aetiologically relevant factors whose effects are cumulative [23]. In contrast to event history techniques, the analysis of the pilot study's data was able to estimate, for a small number of factors, the length of each episode of exposure to a particular factor and, since these episodes are cumulative, the lifetime exposure to that factor.
PILOT STUDY
The pilot study collected retrospective data using the life grid method and indicators based on background routines and analysed these data in terms of cumulative lifetime exposure.
Developing the method The outcome variable of the study was chronic respiratory disease, specifically COAD. COAD was defined in terms of the proportion of total inhalable air which could be expelled in one second or, in spirometric terms, the ratio of the forced vital capacity (FVC) to the one second forced expiratory volume (FEVI). The FVC and FEV~ values were obtained by using a standardized portable spirometer and the highest values achieved in three exhalations were recorded. Separate regression equations for males [24] and females [25] were used to correct for the normal age-related deterioration in lung function. COAD was defined as an FEV]/FVC of more than two standard deviations below the age-adjusted mean. Knowledge of the aetiology of COAD enabled four causal variables to be identified: cigarette smoking; occupational fumes and dusts; atmospheric pollution and residential damp and mould. Two further variables, physically arduous work and inadequate nutrition, neither of which are causally related to COAD, were also included in the study. The indicators of exposure to these factors were developed during in-depth and open-ended interviews with 29 elderly people, most of whom were aged between 70 and 90 years. Of the 30 patients suggested by the general
Table 2. Range of lifetime exposure scores (n = 19) (units are total number of years of exposure to factor) Factor Air pollution Residential damp Physically arduous work Occupational fumes and dusts Inadequate nutrition Cigarette smoking
Range
Mean
Standard deviation
0-35 0-49 0-47
11.2 11.5 10.5
12.1 13.9 12. I
0~9 0-16 0-60
11.9 4.2 30.2
13.0 6.1 19.6
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Table 3. Discriminatorypower of lifetimeexposure scores between COAD cases and non-COAD controls: differencebetween aetiologically relevant and non-relevant factors (units are the number of subjects) Level of exposure Low* Factor Cases Air pollution 4 Residential damp 5 Physically arduous work 6 Occupational fumes and dusts 4 Inadequate nutrition 6 Cigarette smoking 2 *Less than the mean. tBetween the mean and one standard deviation above the mean. *More than one standard deviation above the mean.
practitioners, 29 generously agreed to take part in the study and most of these appeared to enjoy the interview. For four of the factors the resulting indicators are direct, that is directly drawn from the person's recollection of those circumstances. The indicators of residential dampness, physically arduous work, occupational fumes and dusts and cigarette smoking are of this type. The indicators of atmospheric pollution and inadequate nutrition are indirect, that is secondary deductions made by the researcher on the basis of recalled detail. In the case of inadequate nutrition the indirect approach proved necessary because the moral connotations of poverty distorted interviewees' presentation of remembered detail. There exists, however, a series of studies of the standard of living of the British population, stretching from Rowntree [26], through Boyd-Orr [27], Greenwood [28] and Zweig [29] to Townsend [30], which allows one to identify those households at greatest risk of inadequate nutrition, and a rather conservative indirect measure was based on their results. An indirect measure of atmospheric pollution was adopted to take account of the change from coal fires to motor cars as its main source. The indicators were chosen (Table 1) because they have face validity and were found to be accessible from memory. Whether or not exposure occurred is derived from the presence or absence of the relevant indicator. Length of exposure is derived from the life grid. The lifetime exposure score for any particular factor summates the several periods, measured in years, during which exposure to that factor occurred. These lifetime exposure scores can be treated as interval level measures of cumulative exposure, allowing means and standard deviations to be calculated.
Table 4. Discriminatory power of lifetime exposure scores between COAD cases and non-COAD controls: combination of factors Mean number of aetiologically relevant factors to which exposed Level of exposure Moderate plus high levels of exposure High levels of exposure only
Cases
Not cases
2.44 1.44
1.30 0.10
Not cases 7 7 6 7 6 6
Moderatet Cases 1 1 3 2 2 4
Not cases 3 2 2 3 1 4
Highs Cases 4 3 0 3 I 3
Not cases 0 1 2 0 3 0
Testing the method The second part of the pilot work examined the spread and discriminatory power of the lifetime exposure scores. The partners in the collaborating general practice kindly identified a series of 19 patients who represented the full range of C O A D morbidity, ranging from severe C O A D to nonC O A D . All 19 kindly agreed to collaborate with the investigation. Without the interviewer being aware of their C O A D status, these 19 patients were interviewed in their homes. Lifetime exposure scores were established for the six variables of interest and, later, C O A D status was identified by spirometry. Lung function tests were performed after the interviews in order to prevent knowledge of C O A D status from biasing the estimation of lifetime exposures. This attempt to ensure the independence of the measures was least successful with the interviewees suffering from severe C O A D , whose respiratory status was revealed by marked breathlessness at rest and the presence of oxygen condenser, nebulizer and so on. It is difficult to see how to eliminate potential bias when interviewing these subjects. Examining the full range of C O A D status, however, ensured the independence of the measures in most cases. RESULTS The lifetime exposure scores for all six variables were found to have a reasonable range (Table 2). Clustering occurred only at the zero values which indicates that exposure to none of the variables is universal. There are two senses in which the lifetime exposure scores also have reasonable discriminatory power. The means and standard deviations of the lifetime exposure scores were calculated and moderate and high levels of exposure were defined in relation to these (Table 3). Two of the six variables for which lifetime exposure scores were established are not thought to be involved in the aetiology of C O A D . The distribution of C O A D cases on these two variables, physically arduous work and inadequate nutrition, differed from their distribution on the four variables which are involved in the aetiology of C O A D . On the two unrelated variables,
Collecting retrospective data COAD cases are a minority of those exposed to moderate (mean to one standard deviation above the mean) and high (more than one standard deviation above the mean) levels. On the four aetiologically relevant variables, COAD cases are a majority of those exposed to moderate and high levels. The lifetime exposure scores also distinguish between cases of COAD and non-COAD controls. With one exception, COAD cases constitute 100% of those exposed to high levels of the aetiologically relevant variables (Table 3). Finally, COAD cases and non-COAD controls were compared in terms of the number of aetiologically relevant variables to which they had been significantly exposed (Table 4). On average COAD cases had been exposed to 2.4 variables at moderate or high levels, compared with a mean of 1.3 among the controls. If high levels of exposure only are examined, COAD cases had been exposed to a mean of 1.4 variables, compared with a mean of 0.1 among controls.
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residential dampness during childhood may be an especially potent source of chest infections [33, 34]. Of equal interest would be hypotheses concerning specific components of the factors examined in the present pilot study. The anti-oxidants contained in fresh fruit, for example, may have a protective function in relation to COAD, particularly among cigarette smokers [35]. Although the work so far has developed within the perspective of medical sociology and medicine, there is no reason why its use should be confined to these areas. Demographic change will lead to growing research interest in the various aspects of old age. Addressing these issues will often require data which can only be collected retrospectively. Renewed interest in the use of retrospective data is therefore timely. Acknowledgements~rateful thanks are due to the patients
and partners of Forest Road Health Centre who generously collaborated with this research. The paper has greatly benefited from comments on an earlier draft by C. Power and R. Wilkinson.
CONCLUSIONS The results of the pilot study are encouraging and suggest that the method being developed is worth further attention. The technical properties of the measures of lifetime exposure, in the sense of their range and discriminatory power, appear adequate. Examining lifetime exposure in relation to COAD suggests that the method is sound because the results are consistent with existing knowledge of the aetiology of COAD. The results also illustrate the potential usefulness of the method, because they go beyond existing knowledge, or at least its public portrayal, by suggesting the importance of combined exposure to multiple hazards rather than the overwhelming importance of one aetiological agent, cigarette smoking. Blaxter [31] has reported a complex relationship between cigarette smoking, socio-economic circumstances and health, and the results of the pilot study suggest support for her observation. The method needs further development and testing. Archive data from studies conducted several decades ago offer Ca way of achieving this. In some cases [14, 27] the subjects of the original studies have been or could be traced. Current interview data on events from several decades past can be compared with the archive data on the same events. The characteristics of the items which are most likely to be accurately recalled could be further investigated and the hypothesis concerning emotional load more rigorously tested. Further development and testing would justify greater confidence in the method which, in turn, would allow it to be used to examine more subtle hypotheses. Of particular interest would be hypotheses concerning the timing of hazardous exposures. Chest infections during childhood, for example, predispose towards adult COAD [32] and exposure to
REFERENCES
1. Moss L. and Goldstein H. (Eds) The Recall Method in Social Surveys. University of London/Institute of Education, 1979. 2. Blaxter M. Longitudinal studies in Britain relevant to inequalities in health. In Class and Health: Research and Longitudinal Data (Edited by Wilkinson R. G.). Tavistock, London, 1986. 3. Wadsworth M. E. J. The Imprint of Time. Clarendon, Oxford, 1991. 4. Power C., Manor O. and Fox J. Health and Class: The Early Years. Chapman & Hall, London, 1991. 5. Pocock S., Cook D., Shaper A., Phillips A. N. and Walker M. Social class differences in ischaemic heart disease in British men. Lancet ii, 197, 1987. 6. Marmot M., Davey Smith G., Stansfield S., Patel C., North F., Head J., White I., Brunner E. and Feeney A. Health inequalities among British civil servants: the Whitehall II study. Lancet 337, 1387, 1991. 7. Power C. The special role of longitudinal studies. In The Epidemiology of Childhood Disorders (Edited by Pless I.). Oxford University Press, New York, 1993. 8. Baurngarten M., Siemiatycki J. and Gibbs G. W. Validity of work histories obtained by interview for epidemiologic purposes. Am. J. Epidemiol. 118, 583, 1983. 9. Martin J. and Roberts C. Women and Employment: Technical Report. OPCS, London, 1984. 10. Stewart W., Tonascia J. and Matanoski G. The validity of questionnaire reported work histories in live respondents. J. Occup. Med. 29, 795, 1987. 11. Martin C. J. Monitoring maternity services by postal questionnaire: congruity between mothers' reports and their obstetric records. Statistics Med. 6, 613, 1987. 12. Blaxter M. and Paterson E. Mothers and Daughters: A Three Generational Study of Health Attitudes and Behaviour. Heinemann, London, 1982.
13. Conway M., Rubin D., Spinnler H. and Wagenaar W. (Eds) Theoretical Perspectives on Autobiographical Memory. Kluwer Academic Publishers (in collaboration with NATO ScientificAffairs Division), London, 1992. 14. Sheridan D. Writing to the archive: mass-Observation as autobiography. Sociology 27, 27, 1993.
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15. Harrisson T. Living through The Blitz. Collins, London, 1976. 16. Seldon H. and Pappworth J. By Word of Mouth: Elite Oral History. Methuen, London, 1983. 17. Gallie D. The Social Change and Economic Life Initiath~e: An Overview. SCELI Working Paper No. I. Economic and Social Research Council, Swindon, 1988. 18. Elias P. Methodological, statistical and practical issues arising from the collection and analysis of work history information by survey techniques. Bull. method. Sociol. 31, 3, 1991. 19. Allison P. D. Event History Analysis: Regression .]'or Longitudinal Event Data. Sage University Paper. Series: Quantitative Applications in the Social Sciences. Sage, London, 1984. 20. Caselli G., Duchene J., Egidi V., Santini A. and Wunsch G. A matter of life and death: methodologies for the life history analysis of adult mortality. In CICRED (Comite International de Cooperation dans les Recherches Nationales en Demographic). SocioEconomic differential mortality in industrialized countries, No. 7. CICRED, Paris, 1991. 21. Dex S. (Ed.) Life and Work History Analysis: Qualitative and Quantitative Developments. Routledge, London, 1991. 22. Salhi M., Caselli G., Duchene J., Eqidi V., Santini A., Philtqes E. and Wunsch G. Mortality Differentials using Life Histories: a Methodology and its Application to Norwegian Data. IUSSP seminar, University of Messina, June 1992. 23. Fletcher C., Peto R., Tinker C. and Speizer F. The Natural History of Chronic' Bronchitis and Emphysema. Oxford University Press, Oxford, 1976. 24. Berglund E., Birath G., Bjure, Grimby G., Kjellmer I., Sandquist L. and Soderholm B. Spirometric studies in normal subjects. Acta Med. Seand. 173, 185, 1963. 25. Hall A., Heywood C. and Cotes J. Lung function in healthy British women. Thorax 34, 359, 1979. 26. Rowntree B. S. Poverty. Methuen, London, 1901. 27+ Boyd-Orr J. Food, Health and Income: A Survey of Adequacy of Diet in Relation to Income. MacMillan, London, 1936. 28. Greenwood W. How The Other Man Lives. Labour Book Service, London, 1939. 29. Zweig F. Labour, Life and Poverty. Gollanz, London, 1949. 30. Townsend P. Poverty in the United Kingdom. Penguin, London, 1979. 31. Blaxter M. Health and Lifestyles. Tavistock, London, 1990. 32. Mann S., Wadsworth M. and Colley J. Accumulation of factors influencing respiratory illness in members of a national birth cohort and their offspring. J. Epidemiol. Commun. Hhh 46, 286, 1992. 33. Martin C., Platt S. and Hunt S. Housing conditions and health. Br. Med. J. 294, 1125, 1987. 34. Platt S., Martin C., Hunt S. and Lewis C. Damp housing, mould growth and symptomatic health state. Br. Med. J. 298, 1673, 1989. 35. Strachan D., Cox B., Erzinclioglu S. and Walters D. Ventilatory function and winter fresh fruit consumption in a random sample of British adults. Thorax 46, 624, 1991. APPENDIX
The LiJe Grid: Details and Examples The Life Grid, as adapted for use in the pilot study, consisted of four vertical lines on graph paper. The first, left hand, vertical line was the 'external' line which contained the years (1920, 1930, etc.) and important events such as the 1914-18 War, 1926 General Strike, 1939-45 War, 1953 Coronation and 1963 Kennedy assassination. The next
vertical line was the 'personal' line onto which were placed events such as the subject's birth and that of their siblings, departure from the parental home, marriage, birth of children, death of mother and father and death of spouse. The third vertical line was the 'residential' line onto which were placed the residences the subject had occupied, typically a parental home, a series of briefly occupied residences around marriage and early family formation, a main adult residence and, finally, less spacious accommodation in old age or after the death of the spouse. The fourth vertical line was the 'occupational' line onto which were placed the subject's employed occupations, typically a few short term employments during the early years after leaving school, a main adult occupation entered soon after marriage and a final occupation in the ten years before retirement. The details were entered on the life grid in pencil and frequently corrected as subjects improved the accuracy of dating by cross-referencing between the vertical lines. When the Life Grid had been completed, the interview moved on to collect the details of possible exposures. First, the subject's smoking history was obtained. Most subjects started smoking cigarettes when they entered paid employment and were occasional, social smokers through adolescence. Their adult smoking pattern typically started during the war, for men, or at the birth of their first child, for women. The adult smoking pattern continued for three or four decades until the first of several attempts at cessation, usually prompted by poor health or realization of the health-damaging effects of smoking. Interestingly, women who had completed adolescence before World War I were the only subjects who reported never having smoked. Next, subjects were asked for details of each residence which they had occupied. The details collected concerned the indicators of atmospheric pollution and residential dampness which are listed in Table 1, namely, the characteristics of the area in which the residence was situated and the presence or absence of dampness inside the house. Finally, details were collected about each occupation performed; Table 1 lists the indicators which were used for exposure to occupational fumes and dusts and physically arduous work. At the end of the interview the subject's lung function was measured using a portable spirometer. After the interview the subject's lifetime exposure scores were calculated. Whether an exposure had occurred was decided by the presence or absence of the relevant indicators. In the case of inadequate nutrition, the indirect indicator required the combination of information from the personal and occupational lines. The length of exposure, in years, was derived from the life grid. Lifetime exposure to a particular factor was calculated by summating the length of the exposures to the relevant factor.
Examples (I) 77-year-old woman sufJbring Jrom COAD. Spent the first 14 years of her live living in a damp tied cottage in a rural area. Subsequent residences were not damp. From ages 36 to 46 years lived in an urban built-up area, close to main roads and factories. All other residences were in rural or sub-urban areas. No occupational exposure to fumes or dusts. Smoked 3-4 cigarettes per day from ages 14 to 18, 10 per day from 18 to 60 and 2 3 per day from 60 to 77. Lifetime exposure scores (in years): atmospheric pollution 10; residential damp 14; occupational fumes and dusts 0: cigarette smoke 42. (2) 79-year-old woman not suffering from COA D. The first 22 years of her life were spent in built-up urban areas, close to main roads. All other residences were in sub-urban areas. The most recent 10 years of her life were spent in a damp house. All earlier residences had been dry. No occupational exposures to fumes or dust. An 'on-off" cigarette smoker: 10 per day ages 18-21; stopped ages 21-28; 20 per day ages 28-51; stopped ages 51-60; 20 per day ages 60-62; stopped ages 63-79. Lifetime exposure scores (in years): atmospheric
Collecting retrospective data pollution 22; residential damp 10; Occupational fumes and dusts 0; cigarette smoke 29. (3) 77-year-old male suffering from COAD. Spent the first 26 years of his life in urban built-up areas, close to rail tracks and factories. The rest of his life was spent in sub-urban areas. The residences in which he spent the first 26 years of his life were also damp, as were the residences in which he lived at ages 42 50 and 65 72. All other residences were dry. He was exposed to a range of chemicals, including cyanide and sulphuric acid, at ages 23-42 when he worked in the electroplating industry and to D D T and formaldehyde at ages 55 65 when he worked as a household vermin exterminator. Some of his other occupations, such as truck driver, d u s t m a n and highway tar-layer, were dirty but out-of-doors. He smoked I0 cigarettes per day at ages 14~40. Lifetime
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exposure scores (in years): atmospheric pollution 26; residential dampness 34; occupational fumes and dusts 29; cigarette smoke 26. (4) 78-year-old man not suffering from COAD. He had spent his whole life in sub-urban areas. During his life he had occupied two residences, neither of which was damp. He worked his whole life in the Post Office, starting as a porter and storeman, then in various office grades at ages 40-~0, after which he took early retirement. He was never occupationally exposed to fumes or dusts. He started smoking the occasional cigarette at age 21; smoked 10 per day at ages 25 35 and 20 per day at ages 3 5 ~ 0 , after which he stopped. Lifetime exposure scores (in years): atmospheric pollution 0; residential dampness 0; occupational fumes and dusts 0; cigarette smoke 35.
Soc. Sci, Med. Vol. 42, No. 5, pp. 751-757, 1996
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RETROSPECTIVE
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D. B. BLANE Academic Department of Psychiatry, Charing Cross and Westminster Medical School. London W6 8RP, England Abstract--The present paper argues that a need will remain for data which have been collected retrospectively. Recent developments in oral history and sociology are described to suggest a method of collecting retrospective data which may minimize recall bias. Pilot work is reported on the adaptation of the method for a study of chronic respiratory disease. The results of the pilot study proved promising. The technical properties of the measures appeared adequate. The results of the substantive analysis were consistent with existing knowledge and went beyond existing knowledge to suggest new areas of research. Ways of further validating the method are identified and its wider application discussed.
Key words--retrospective data, life grid, lifetime exposure, chronic respiratory disease
INTRODUCTION The present paper reports the early stages of a program of work designed to develop a method of studying the development and causes of chronic disease. The pilot work has been concerned with one chronic disease, namely, chronic obstructive airway disease (COAD), which unites the previously distinct diagnoses of emphysema and chronic bronchitis. In common with the 'new epidemiology', the work draws on the ideas of both the natural and social sciences and integrates qualitative and quantitative methods. It is intended to help answer patients' questions about the causes of their disease, an area where at present medicine too often has to confess its ignorance. Those studying the origins and development of chronic diseases usually share the view that retrospective data are unreliable and subject to significant recall bias [1]. Recall bias is seen as particularly hazardous to the study of chronic disease, because these diseases often develop slowly over several decades. There is adequate time for the subjective re-working and re-interpretation of past events as individuals attempt to explain the calamity that has befallen them. Longitudinal studies eliminate most of the problems of recall bias and have become the preferred method of study in this area. At present there are at least 20 such studies under way in Britain 12]. Before describing the pilot work and its results, the paper examines the limitations of longitudinal studies and suggests that a need remains for data which have been collected retrospectively. The longitudinal and retrospective approaches are not seen as competitors. ssr~42/s---t
Both approaches are essential and they complement one another. The present paper is concerned with the retrospective approach and identifies recent developments which may minimize the unreliability of data collected in this way.
LIMITATIONS OF LONGITUDINALSTUDIES In relation to chronic disease, practical issues limit the usefulness of longitudinal studies. The most powerful longitudinal study tracks a birth cohort from birth through life. The earliest birth cohorts in Britain were born in 1946 [3] and 1958 [4] and the members of these cohorts are presently aged in their mid-40s and mid-30s respectively. At these ages most of the prevalent chronic diseases are in the early stages of their natural history and manifest disease is unusual. Chronic diseases develop over many decades, but in most cases they do not become manifest as overt pathology until late middle age or older. By the time the chronic diseases in these cohort members reach the clinically significant part of their natural history, the present generation of researchers will have long retired. Because birth cohorts take a long time to yield results in this area, the need remains for methods of study which can produce answers more quickly. Some studies attempt to avoid this problem by starting at later age groups where chronic diseases can be expected to become common during the first 10 or 20 years of follow-up [5, 6]. Such studies necessarily remain blind to events in the earlier part of life or they have to use retrospective methods to obtain this information. 751
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Sample attrition is a second limitation of longitudinal studies. Careful management can reduce the attrition rate to remarkably low levels, but longitudinal studies nevertheless lose a proportion of their sample, and this proportion tends to increase over time. Losses occur for a number of reasons, among the most important of which are refusal to further participate in the study and high levels of geographic mobility which make subjects difficult to trace. Losses to the original sample are likely to be socially selected and Granada Television's Seven-Up, which can be seen as a TV longitudinal study, suggests the nature of this selection. By the fourth sweep, at age 28, three of the original 15 subjects had dropped-out of the study. The two who refused to participate further were members of the service class, a barrister and a BBC executive. The third was geographically mobile, and so hard to trace, was unemployed and in poor health. Data from academic studies confirm this impression. Losses to the National Survey of Health and Development at age 15, for example, came disproportionately from the extremes of the social hierarchy, on the one hand subjects whose parents were owner-occupiers and possessed high educational qualifications, on the other hand those living in overcrowded housing. By age 36, those with low educational qualifications and those suffering from a psychiatric illness had also been disproportionately lost to the study [3]. Attrition to the National Child Development Study reduced the sample to 79.5% by age 16, with those from disadvantaged groups being most likely to be lost to follow-up [7]. This problem of selective attrition is not unique to longitudinal studies; it equally affects cross-sectional studies, where it is caused by similar factors. Third, longitudinal studies are vulnerable to the emergence of new research questions. The data which are collected in any particular sweep are relevant to the research questions of that decade. No study can predict which information about, say, 15 year olds will be relevant to the research questions that are current 30 years later. The missing data may be of one type, such as a particular measure of health, or cover a whole field of enquiry, such as the quality of material life. If current research interests requires such missing data, then their collection will probably depend on the retrospective method. Similarly, because results from longitudinal studies are not quickly available, by the time earlier conditions are evaluated they may be less relevant to current circumstances due to changes occurring in the intervening period. Practicality, doubts about representativeness and the emergence of new research questions all suggest a continuing need for data which have been collected retrospectively. To be useful, the method of their collection needs to minimize the well-documented sources of their unreliability [l]. Recent developments in oral history and sociology suggest two ways in which this might be achieved.
CHARACTERISTICS OF ACCURATE RECALL
Awareness of the unreliability of retrospective data has diverted attention from the potential accuracy of longterm memory. Studies which compare recalled basic occupational information against official records, for example, typically find around 80% agreement between these two sources [8-10]. A comparison of maternity records and mothers' accounts of the same events found similarly high levels of agreement on many items [11]. A study which compared recalled information obtained by contemporary interviews with medical and other records from 30 years earlier, also found high levels of agreement between the recorded and the recalled versions; on the items concerning breast feeding, for example, there were no discrepancies at all in 19 out of the 21 cases [12]. The studies cited above required specific information about discrete events. If equally reliable data about the more general aspects of life are to be obtained, it would be useful to know which types of event are remembered most accurately. Tests of recall after several months suggest that its accuracy is influenced by the emotional load attached to a particular item of information. Emotionally laden events are least likely to be recalled acccurately; in an extreme example, neonatal resuscitation was recalled less accurately than whether the child had been breastfed [11}. The emotional load carried by an event may also be important to memory over the longer periods of time relevant to the natural history of chronic disease [13]. Volunteer civilians recorded the details of life for Mass Observation during World War I! [14]. These records often differ from present day accounts of the same events by the same individuals, particularly where the events being recalled were traumatic, such as a bombing raid [! 5]. Conversely, hum-drum events which carry little emotional charge and the barely noticed background routines of life appear to be recalled most accurately [16]. Attempts to collect complex data retrospectively may benefit from avoiding items of information which are emotionally loaded and could usefully concentrate on the background, hum-drum details of life. The comparative accuracy with which emotionally neutral items are recalled has an interesting implication for the study of chronic respiratory disease. Public health education and the advice of doctors during consultations attach overwhelming importance to cigarette smoking as the cause of respiratory disease and pay comparatively little attention to its other known causes. The subjects of the presently reported study, for example, were often puzzled by questions about factors other than cigarette smoking. If events which are publicly accorded little importance tend to be recalled more accurately than those which are singled out for public attention, then items about occupational fumes and dusts, atmospheric
Collecting retrospective data
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Table I. Variables and their indicators Variable
Type of measure
Indicators
Residential damp
Indirect via population density Direct
Physically arduous work
Direct
Occupational fumes and dusts
Direct
Inadequate nutrition
Indirect. via household structure and income Direct
Continuously built-up area with most buildings at least three stories above ground level Wallpaper peels off walls; mould grows on internal walls; clothes steam when aired after storage Work perceived as heavy: heavy sweating; back injuries frequent among workforce Nature of work process; site (e.g. basement); level of ventilation (e.g. out-of-doors, extractor fans) One social class IV or V income coming into a household containing more than two children Smoking five or more cigarettes per day
Air pollution
Cigarette smoking
pollution and residential damp may be recalled more accurately by those suffering from chronic respiratory disease than the details of their cigarette smoking. The Social Change and Economic Life study has contributed a further advance in methods by developing the Life Grid [17, 18]. A full description of this method has yet to be published, but it was described and much discussed during the BSA annual conference on sociology and history in 1988. The method involves cross-referencing the dates of any changes in the areas of interest, for example occupation and housing, against dates in the subject's personal life, such as marriage and death of mother, as well as against events in the external world, like coronations and wars. Further details and examples are contained in the Appendix. Cross-referencing on the life grid enables subjects to improve the accuracy with which dates are remembered, which allows better estimates of the duration of events to be calculated. The life grid also appears to release detail from memory by juxtaposing different information from the same period of life. One subject, for example, was surprised to be able to list the chemicals he had used when employed as a chromium plater before World War II (nickel, chromium, copper sulphate, cyanide, sulphuric acid and caustic soda), something he claimed not to have thought about for 40 years. The data collected by the present study could have been analysed using event history techniques [18-22]. Event history analysis is concerned with changes in status, such as that from employed to unemployed, and calculates the probability that a specific change will be associated with a given outcome, such as increased mortality. A simpler method of analysis could be used in the present pilot study because a small number of subjects were involved and because previous research on respiratory disease pointed to a small number of aetiologically relevant factors whose effects are cumulative [23]. In contrast to event history techniques, the analysis of the pilot study's data was able to estimate, for a small number of factors, the length of each episode of exposure to a particular factor and, since these episodes are cumulative, the lifetime exposure to that factor.
PILOT STUDY
The pilot study collected retrospective data using the life grid method and indicators based on background routines and analysed these data in terms of cumulative lifetime exposure.
Developing the method The outcome variable of the study was chronic respiratory disease, specifically COAD. COAD was defined in terms of the proportion of total inhalable air which could be expelled in one second or, in spirometric terms, the ratio of the forced vital capacity (FVC) to the one second forced expiratory volume (FEVI). The FVC and FEV~ values were obtained by using a standardized portable spirometer and the highest values achieved in three exhalations were recorded. Separate regression equations for males [24] and females [25] were used to correct for the normal age-related deterioration in lung function. COAD was defined as an FEV]/FVC of more than two standard deviations below the age-adjusted mean. Knowledge of the aetiology of COAD enabled four causal variables to be identified: cigarette smoking; occupational fumes and dusts; atmospheric pollution and residential damp and mould. Two further variables, physically arduous work and inadequate nutrition, neither of which are causally related to COAD, were also included in the study. The indicators of exposure to these factors were developed during in-depth and open-ended interviews with 29 elderly people, most of whom were aged between 70 and 90 years. Of the 30 patients suggested by the general
Table 2. Range of lifetime exposure scores (n = 19) (units are total number of years of exposure to factor) Factor Air pollution Residential damp Physically arduous work Occupational fumes and dusts Inadequate nutrition Cigarette smoking
Range
Mean
Standard deviation
0-35 0-49 0-47
11.2 11.5 10.5
12.1 13.9 12. I
0~9 0-16 0-60
11.9 4.2 30.2
13.0 6.1 19.6
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Table 3. Discriminatorypower of lifetimeexposure scores between COAD cases and non-COAD controls: differencebetween aetiologically relevant and non-relevant factors (units are the number of subjects) Level of exposure Low* Factor Cases Air pollution 4 Residential damp 5 Physically arduous work 6 Occupational fumes and dusts 4 Inadequate nutrition 6 Cigarette smoking 2 *Less than the mean. tBetween the mean and one standard deviation above the mean. *More than one standard deviation above the mean.
practitioners, 29 generously agreed to take part in the study and most of these appeared to enjoy the interview. For four of the factors the resulting indicators are direct, that is directly drawn from the person's recollection of those circumstances. The indicators of residential dampness, physically arduous work, occupational fumes and dusts and cigarette smoking are of this type. The indicators of atmospheric pollution and inadequate nutrition are indirect, that is secondary deductions made by the researcher on the basis of recalled detail. In the case of inadequate nutrition the indirect approach proved necessary because the moral connotations of poverty distorted interviewees' presentation of remembered detail. There exists, however, a series of studies of the standard of living of the British population, stretching from Rowntree [26], through Boyd-Orr [27], Greenwood [28] and Zweig [29] to Townsend [30], which allows one to identify those households at greatest risk of inadequate nutrition, and a rather conservative indirect measure was based on their results. An indirect measure of atmospheric pollution was adopted to take account of the change from coal fires to motor cars as its main source. The indicators were chosen (Table 1) because they have face validity and were found to be accessible from memory. Whether or not exposure occurred is derived from the presence or absence of the relevant indicator. Length of exposure is derived from the life grid. The lifetime exposure score for any particular factor summates the several periods, measured in years, during which exposure to that factor occurred. These lifetime exposure scores can be treated as interval level measures of cumulative exposure, allowing means and standard deviations to be calculated.
Table 4. Discriminatory power of lifetime exposure scores between COAD cases and non-COAD controls: combination of factors Mean number of aetiologically relevant factors to which exposed Level of exposure Moderate plus high levels of exposure High levels of exposure only
Cases
Not cases
2.44 1.44
1.30 0.10
Not cases 7 7 6 7 6 6
Moderatet Cases 1 1 3 2 2 4
Not cases 3 2 2 3 1 4
Highs Cases 4 3 0 3 I 3
Not cases 0 1 2 0 3 0
Testing the method The second part of the pilot work examined the spread and discriminatory power of the lifetime exposure scores. The partners in the collaborating general practice kindly identified a series of 19 patients who represented the full range of C O A D morbidity, ranging from severe C O A D to nonC O A D . All 19 kindly agreed to collaborate with the investigation. Without the interviewer being aware of their C O A D status, these 19 patients were interviewed in their homes. Lifetime exposure scores were established for the six variables of interest and, later, C O A D status was identified by spirometry. Lung function tests were performed after the interviews in order to prevent knowledge of C O A D status from biasing the estimation of lifetime exposures. This attempt to ensure the independence of the measures was least successful with the interviewees suffering from severe C O A D , whose respiratory status was revealed by marked breathlessness at rest and the presence of oxygen condenser, nebulizer and so on. It is difficult to see how to eliminate potential bias when interviewing these subjects. Examining the full range of C O A D status, however, ensured the independence of the measures in most cases. RESULTS The lifetime exposure scores for all six variables were found to have a reasonable range (Table 2). Clustering occurred only at the zero values which indicates that exposure to none of the variables is universal. There are two senses in which the lifetime exposure scores also have reasonable discriminatory power. The means and standard deviations of the lifetime exposure scores were calculated and moderate and high levels of exposure were defined in relation to these (Table 3). Two of the six variables for which lifetime exposure scores were established are not thought to be involved in the aetiology of C O A D . The distribution of C O A D cases on these two variables, physically arduous work and inadequate nutrition, differed from their distribution on the four variables which are involved in the aetiology of C O A D . On the two unrelated variables,
Collecting retrospective data COAD cases are a minority of those exposed to moderate (mean to one standard deviation above the mean) and high (more than one standard deviation above the mean) levels. On the four aetiologically relevant variables, COAD cases are a majority of those exposed to moderate and high levels. The lifetime exposure scores also distinguish between cases of COAD and non-COAD controls. With one exception, COAD cases constitute 100% of those exposed to high levels of the aetiologically relevant variables (Table 3). Finally, COAD cases and non-COAD controls were compared in terms of the number of aetiologically relevant variables to which they had been significantly exposed (Table 4). On average COAD cases had been exposed to 2.4 variables at moderate or high levels, compared with a mean of 1.3 among the controls. If high levels of exposure only are examined, COAD cases had been exposed to a mean of 1.4 variables, compared with a mean of 0.1 among controls.
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residential dampness during childhood may be an especially potent source of chest infections [33, 34]. Of equal interest would be hypotheses concerning specific components of the factors examined in the present pilot study. The anti-oxidants contained in fresh fruit, for example, may have a protective function in relation to COAD, particularly among cigarette smokers [35]. Although the work so far has developed within the perspective of medical sociology and medicine, there is no reason why its use should be confined to these areas. Demographic change will lead to growing research interest in the various aspects of old age. Addressing these issues will often require data which can only be collected retrospectively. Renewed interest in the use of retrospective data is therefore timely. Acknowledgements~rateful thanks are due to the patients
and partners of Forest Road Health Centre who generously collaborated with this research. The paper has greatly benefited from comments on an earlier draft by C. Power and R. Wilkinson.
CONCLUSIONS The results of the pilot study are encouraging and suggest that the method being developed is worth further attention. The technical properties of the measures of lifetime exposure, in the sense of their range and discriminatory power, appear adequate. Examining lifetime exposure in relation to COAD suggests that the method is sound because the results are consistent with existing knowledge of the aetiology of COAD. The results also illustrate the potential usefulness of the method, because they go beyond existing knowledge, or at least its public portrayal, by suggesting the importance of combined exposure to multiple hazards rather than the overwhelming importance of one aetiological agent, cigarette smoking. Blaxter [31] has reported a complex relationship between cigarette smoking, socio-economic circumstances and health, and the results of the pilot study suggest support for her observation. The method needs further development and testing. Archive data from studies conducted several decades ago offer Ca way of achieving this. In some cases [14, 27] the subjects of the original studies have been or could be traced. Current interview data on events from several decades past can be compared with the archive data on the same events. The characteristics of the items which are most likely to be accurately recalled could be further investigated and the hypothesis concerning emotional load more rigorously tested. Further development and testing would justify greater confidence in the method which, in turn, would allow it to be used to examine more subtle hypotheses. Of particular interest would be hypotheses concerning the timing of hazardous exposures. Chest infections during childhood, for example, predispose towards adult COAD [32] and exposure to
REFERENCES
1. Moss L. and Goldstein H. (Eds) The Recall Method in Social Surveys. University of London/Institute of Education, 1979. 2. Blaxter M. Longitudinal studies in Britain relevant to inequalities in health. In Class and Health: Research and Longitudinal Data (Edited by Wilkinson R. G.). Tavistock, London, 1986. 3. Wadsworth M. E. J. The Imprint of Time. Clarendon, Oxford, 1991. 4. Power C., Manor O. and Fox J. Health and Class: The Early Years. Chapman & Hall, London, 1991. 5. Pocock S., Cook D., Shaper A., Phillips A. N. and Walker M. Social class differences in ischaemic heart disease in British men. Lancet ii, 197, 1987. 6. Marmot M., Davey Smith G., Stansfield S., Patel C., North F., Head J., White I., Brunner E. and Feeney A. Health inequalities among British civil servants: the Whitehall II study. Lancet 337, 1387, 1991. 7. Power C. The special role of longitudinal studies. In The Epidemiology of Childhood Disorders (Edited by Pless I.). Oxford University Press, New York, 1993. 8. Baurngarten M., Siemiatycki J. and Gibbs G. W. Validity of work histories obtained by interview for epidemiologic purposes. Am. J. Epidemiol. 118, 583, 1983. 9. Martin J. and Roberts C. Women and Employment: Technical Report. OPCS, London, 1984. 10. Stewart W., Tonascia J. and Matanoski G. The validity of questionnaire reported work histories in live respondents. J. Occup. Med. 29, 795, 1987. 11. Martin C. J. Monitoring maternity services by postal questionnaire: congruity between mothers' reports and their obstetric records. Statistics Med. 6, 613, 1987. 12. Blaxter M. and Paterson E. Mothers and Daughters: A Three Generational Study of Health Attitudes and Behaviour. Heinemann, London, 1982.
13. Conway M., Rubin D., Spinnler H. and Wagenaar W. (Eds) Theoretical Perspectives on Autobiographical Memory. Kluwer Academic Publishers (in collaboration with NATO ScientificAffairs Division), London, 1992. 14. Sheridan D. Writing to the archive: mass-Observation as autobiography. Sociology 27, 27, 1993.
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15. Harrisson T. Living through The Blitz. Collins, London, 1976. 16. Seldon H. and Pappworth J. By Word of Mouth: Elite Oral History. Methuen, London, 1983. 17. Gallie D. The Social Change and Economic Life Initiath~e: An Overview. SCELI Working Paper No. I. Economic and Social Research Council, Swindon, 1988. 18. Elias P. Methodological, statistical and practical issues arising from the collection and analysis of work history information by survey techniques. Bull. method. Sociol. 31, 3, 1991. 19. Allison P. D. Event History Analysis: Regression .]'or Longitudinal Event Data. Sage University Paper. Series: Quantitative Applications in the Social Sciences. Sage, London, 1984. 20. Caselli G., Duchene J., Egidi V., Santini A. and Wunsch G. A matter of life and death: methodologies for the life history analysis of adult mortality. In CICRED (Comite International de Cooperation dans les Recherches Nationales en Demographic). SocioEconomic differential mortality in industrialized countries, No. 7. CICRED, Paris, 1991. 21. Dex S. (Ed.) Life and Work History Analysis: Qualitative and Quantitative Developments. Routledge, London, 1991. 22. Salhi M., Caselli G., Duchene J., Eqidi V., Santini A., Philtqes E. and Wunsch G. Mortality Differentials using Life Histories: a Methodology and its Application to Norwegian Data. IUSSP seminar, University of Messina, June 1992. 23. Fletcher C., Peto R., Tinker C. and Speizer F. The Natural History of Chronic' Bronchitis and Emphysema. Oxford University Press, Oxford, 1976. 24. Berglund E., Birath G., Bjure, Grimby G., Kjellmer I., Sandquist L. and Soderholm B. Spirometric studies in normal subjects. Acta Med. Seand. 173, 185, 1963. 25. Hall A., Heywood C. and Cotes J. Lung function in healthy British women. Thorax 34, 359, 1979. 26. Rowntree B. S. Poverty. Methuen, London, 1901. 27+ Boyd-Orr J. Food, Health and Income: A Survey of Adequacy of Diet in Relation to Income. MacMillan, London, 1936. 28. Greenwood W. How The Other Man Lives. Labour Book Service, London, 1939. 29. Zweig F. Labour, Life and Poverty. Gollanz, London, 1949. 30. Townsend P. Poverty in the United Kingdom. Penguin, London, 1979. 31. Blaxter M. Health and Lifestyles. Tavistock, London, 1990. 32. Mann S., Wadsworth M. and Colley J. Accumulation of factors influencing respiratory illness in members of a national birth cohort and their offspring. J. Epidemiol. Commun. Hhh 46, 286, 1992. 33. Martin C., Platt S. and Hunt S. Housing conditions and health. Br. Med. J. 294, 1125, 1987. 34. Platt S., Martin C., Hunt S. and Lewis C. Damp housing, mould growth and symptomatic health state. Br. Med. J. 298, 1673, 1989. 35. Strachan D., Cox B., Erzinclioglu S. and Walters D. Ventilatory function and winter fresh fruit consumption in a random sample of British adults. Thorax 46, 624, 1991. APPENDIX
The LiJe Grid: Details and Examples The Life Grid, as adapted for use in the pilot study, consisted of four vertical lines on graph paper. The first, left hand, vertical line was the 'external' line which contained the years (1920, 1930, etc.) and important events such as the 1914-18 War, 1926 General Strike, 1939-45 War, 1953 Coronation and 1963 Kennedy assassination. The next
vertical line was the 'personal' line onto which were placed events such as the subject's birth and that of their siblings, departure from the parental home, marriage, birth of children, death of mother and father and death of spouse. The third vertical line was the 'residential' line onto which were placed the residences the subject had occupied, typically a parental home, a series of briefly occupied residences around marriage and early family formation, a main adult residence and, finally, less spacious accommodation in old age or after the death of the spouse. The fourth vertical line was the 'occupational' line onto which were placed the subject's employed occupations, typically a few short term employments during the early years after leaving school, a main adult occupation entered soon after marriage and a final occupation in the ten years before retirement. The details were entered on the life grid in pencil and frequently corrected as subjects improved the accuracy of dating by cross-referencing between the vertical lines. When the Life Grid had been completed, the interview moved on to collect the details of possible exposures. First, the subject's smoking history was obtained. Most subjects started smoking cigarettes when they entered paid employment and were occasional, social smokers through adolescence. Their adult smoking pattern typically started during the war, for men, or at the birth of their first child, for women. The adult smoking pattern continued for three or four decades until the first of several attempts at cessation, usually prompted by poor health or realization of the health-damaging effects of smoking. Interestingly, women who had completed adolescence before World War I were the only subjects who reported never having smoked. Next, subjects were asked for details of each residence which they had occupied. The details collected concerned the indicators of atmospheric pollution and residential dampness which are listed in Table 1, namely, the characteristics of the area in which the residence was situated and the presence or absence of dampness inside the house. Finally, details were collected about each occupation performed; Table 1 lists the indicators which were used for exposure to occupational fumes and dusts and physically arduous work. At the end of the interview the subject's lung function was measured using a portable spirometer. After the interview the subject's lifetime exposure scores were calculated. Whether an exposure had occurred was decided by the presence or absence of the relevant indicators. In the case of inadequate nutrition, the indirect indicator required the combination of information from the personal and occupational lines. The length of exposure, in years, was derived from the life grid. Lifetime exposure to a particular factor was calculated by summating the length of the exposures to the relevant factor.
Examples (I) 77-year-old woman sufJbring Jrom COAD. Spent the first 14 years of her live living in a damp tied cottage in a rural area. Subsequent residences were not damp. From ages 36 to 46 years lived in an urban built-up area, close to main roads and factories. All other residences were in rural or sub-urban areas. No occupational exposure to fumes or dusts. Smoked 3-4 cigarettes per day from ages 14 to 18, 10 per day from 18 to 60 and 2 3 per day from 60 to 77. Lifetime exposure scores (in years): atmospheric pollution 10; residential damp 14; occupational fumes and dusts 0: cigarette smoke 42. (2) 79-year-old woman not suffering from COA D. The first 22 years of her life were spent in built-up urban areas, close to main roads. All other residences were in sub-urban areas. The most recent 10 years of her life were spent in a damp house. All earlier residences had been dry. No occupational exposures to fumes or dust. An 'on-off" cigarette smoker: 10 per day ages 18-21; stopped ages 21-28; 20 per day ages 28-51; stopped ages 51-60; 20 per day ages 60-62; stopped ages 63-79. Lifetime exposure scores (in years): atmospheric
Collecting retrospective data pollution 22; residential damp 10; Occupational fumes and dusts 0; cigarette smoke 29. (3) 77-year-old male suffering from COAD. Spent the first 26 years of his life in urban built-up areas, close to rail tracks and factories. The rest of his life was spent in sub-urban areas. The residences in which he spent the first 26 years of his life were also damp, as were the residences in which he lived at ages 42 50 and 65 72. All other residences were dry. He was exposed to a range of chemicals, including cyanide and sulphuric acid, at ages 23-42 when he worked in the electroplating industry and to D D T and formaldehyde at ages 55 65 when he worked as a household vermin exterminator. Some of his other occupations, such as truck driver, d u s t m a n and highway tar-layer, were dirty but out-of-doors. He smoked I0 cigarettes per day at ages 14~40. Lifetime
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exposure scores (in years): atmospheric pollution 26; residential dampness 34; occupational fumes and dusts 29; cigarette smoke 26. (4) 78-year-old man not suffering from COAD. He had spent his whole life in sub-urban areas. During his life he had occupied two residences, neither of which was damp. He worked his whole life in the Post Office, starting as a porter and storeman, then in various office grades at ages 40-~0, after which he took early retirement. He was never occupationally exposed to fumes or dusts. He started smoking the occasional cigarette at age 21; smoked 10 per day at ages 25 35 and 20 per day at ages 3 5 ~ 0 , after which he stopped. Lifetime exposure scores (in years): atmospheric pollution 0; residential dampness 0; occupational fumes and dusts 0; cigarette smoke 35.