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Variations between schools and regions in smokingprevalence among British schoolchildren-implications for health education
Publ. Hlth, Lond (1985) 99, 243-249
Variations between Schools and Regions in Smoking Prevalence among British Schoolchildren- Implications for Health Education Anne Charlton Cancer ResearchCampaign Fellow
Manchester Regional Committee for Cancer Education Kinnaird Road, Manchester M20 9QL Pam Gillies Lecturer
Department of Community Health, University of Nottingham and Frank L e d w i t h Research Fellow
Department of Education, University of Manchester
Surveys of adolescents' smoking habits in three regions, the north of England, Sheffield and the east of Scotland were conducted concurrently with a national survey (O.P.C.S.) in late autumn 1982. Considerable variations were found in smoking prevalence between regions and also between individual schools. Since the timing of school education about smoking has been found to be critical, and the conclusion drawn suggests that the national data on smoking prevalence is not a sufficiently precise guide, each school may need to conduct its own survey to estimate numbers of children in each age group who have tried smoking or have already become regular smokers. Introduction A recent government national survey 1 has highlighted the extent of the smoking problem among adolescents, A quarter of all secondary schoolchildren are regular smokers by the time they leave school, and adolescent smokers as a whole are smoking an estimated 18 to 24 million cigarettes per week in England and Wales and 2.2-2.8 million in Scotland, on which they are spending well over a million pounds. Within the field of preventive medicine there can therefore be no doubt that anti-smoking education still has a high priority. The timing of school health education about smoking appears to be critical. Too early an introduction m a y lead to increased experimentation as has been shown with other drug education, ~ whilst school interventions have been found to have less impact on those who are already smoking regularly, a, 4 These findings have suggested that it may be important to intervene before the behaviour has become habitual, but not until some of the children have begun to experiment with smoking. In order to do this we need to know when this stage is reached. The findings of the national survey of the Office of Population Censuses and Surveys (O.P.C,S.) suggested that regional and school 0033-3506/85/040243 +07 $02.00/0
© 1985 The Society of Community Medicine
244
A. Charlton et al.
differences could exist. However the report of that survey noted that the study's sampling frame (with small random samples from within individual schools) was designed to estimate national prevalence data but could not be used to estimate differences between schools and could only give a broad indication of regional differences. The surveys described in this report were intended to provide large enough samples, surveyed by standardized methods, to enable regions and individual schools to be compared. Methods
The three surveys covered the north of England: all Cumbria and Tyne and Wear education authorities (65 schools and colleges of further education, sample size: 15,709); Sheffield (10 schools, sample size, 4398); and Scotland (Lothian, Tayside and Grampian education authorities, 13 groups, i.e. a secondary school and its feeder primary schools, sample size, 4897). Although the northern sample included two fee-paying schools, only state primary, middle and secondary schools were included in the analysis described in this paper. Random samples of schools were drawn to provide 10 ~ and 5 ~ samples of children on school rolls in the North and Sheffield respectively. In Scotland a probability sample of secondary schools was drawn within each local authority area. For each high school selected a probability sample was drawn of two feeder primary schools. In the North and Sheffield the sampling frame included all children aged 9 years and upwards in the selected schools, whilst in Scotland only the children in the last year of primary and first year of secondary schools were tested. In all three regions all children in the target year groupings were given the questionnaire. No attempt was made to follow up absentees. For the purposes of this paper, only the 9-16 years age group is described in order to maintain uniformity since students aged 16 years and over may diverge from patterns of full-time compulsory education. Every effort was made to minimize under-reporting of smoking. The questionnaires were anonymous and administered to intact classes which has been found to give the highest reported prevalence. 5 To protect anonymity and thus encourage truthful answers in a test supervised by the class teacher, an envelope was provided into which each individual pupil sealed his or her script on completion. The basic question on smoking behaviour was the same as that used in the O.P.C.S. national survey and asked the children to place themselves within one of six categories ranging from " I have never smoked" to " I usually smoke more than six cigarettes per week". In addition to describing the regular smoker by the standard definition of those who smoked at least one cigarette per week, a n " ever smoked" category was used which included both the current smokers, those who used to smoke and those who had experimented with smoking. The "ever smoked" category may be the more useful one for purposes of deciding the age group at which to introduce anti-smoking education within schools. The surveys were carried out in the late autumn 1982 at the same time as the national survey. The results were analysed for all three regions, using the Statistical Package of the Social Sciences (SPSS) and by Generalized Linear Interactive Modelling (GLIM).
245
Smoking among British Schoolchildren
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Smoking among British Schoolchildren
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TABLE3. Percentages of children who had ever smoked a cigarette in state secondary schools with highest and lowest smoking prevalence in each survey Secondary School Year Groups 1 (Eng.) 2 (Eng.); 1(Scot.) 11-12 years 12-13 years School average Boys
3 (Eng.) 13-14 years
4 (Eng.) 14-15 years
5 (Eng.) 15-16 years
Girls
B o y s Girls
B o y s Girls
B o y s Girls
B o y s Girls
North "Highest school" Base "Lowest school" Base Sheffield "Highest school" % Base "Lowest school" Base Scotland "Highest school" % Base "Lowest school" Base
67 (273)
58 (31)
52 (25)
66 (21)
50 (30)
77 (26)
82 (22)
75 (32)
65 (26)
74 (35)
68 (25)
50 (554)
20 (59)
19 (54)
49 (65)
40 (43)
51 (53)
50 (40)
59 (54)
65 (63)
70 (63)
63 (60)
54 (845)
42 (96)
35 (89)
46 (87)
27 (78)
61 (93)
67 (97)
68 (86)
67 (85)
68 (80)
59 (54)
47 (674)
42 (69)
21 (71)
45 (76)
32 (59)
45 (74)
54 (55)
65 (84)
62 (71)
58 (52)
48 (63)
57 (185)
---
---
63 (102)
51 (83)
. .
. .
. .
. .
. .
. .
30 (139)
---
---
34 (71)
27 (68)
. .
. .
. .
. .
. .
. .
Results 1. Regional differences
Prevalence of regular smoking A l t h o u g h the increasing trend in prevalence of s m o k i n g with increasing age was similar in all three regions, differences were observed between regions at specific ages, as shown in T a b l e 1. Especially striking was the greater prevalence o f s m o k i n g a m o n g s t boys aged 14-15 years (X2 = 14-55; d.f. 1; P < 0.001) a n d boys 15-16 years (Xz = 4.26; d.f. 1 ; P < 0.05) in the N o r t h c o m p a r e d with those in the Sheffield sample. The O.P.C.S. n a t i o n a l survey findings for the c o u n t r y as a whole resembled those of the N o r t h m o r e closely t h a n those of Sheffield. The suggestion m a d e in the n a t i o n a l survey report that regional differences could exist w o u l d therefore seem to be b o r n e out by these findings.
Prevalence o f " ever smoked" As in the case of regular smoking, t h e " ever s m o k e d " category increased in prevalence with increasing age, b u t obviously at a generally higher level (Table 2). T h e r e were, however, few differences between regions in this category, the only statistically significant one occurring in the oldest age group, 15-16 years, as follows (boys, X2 = 5.70; d.f. 1 ; P < 0.02: girls Xz = 19.13; d.f. 1; P < 0-0001).
248
A. Charlton et al.
By the age of 11-12 years, a quarter of the children had at least tried a cigarette once and by the age of 15-16, almost three-quarters of the children in the North and approximately two-thirds in Sheffield had tried smoking. 2. School differences
Although the overall prevalence for "ever smoked" was basically similar for all regions, very considerable differences were observed between individual schools (X2 = 179.47; d.f. 15; P < 0.001 when the standard Chi-Square test was used on all the secondary schools in which first to fifth years were included in the survey, i.e. in the North and Sheffield). GLIM analyses also showed significant differences between schools in each region. In the North, as Table 3 shows, the widest variations between schools lay in the youngest age groups with the gap narrowed with increasing age, which might perhaps suggest a possible "carry-over" from primary schools which diminishes as the children become established at secondary school. However this effect was not observed in Sheffield schools, although it was perhaps modified to some extent by the "middle school" system. In the Scottish schools there was a clear carry-over effect; the Pearson correlation between smoking prevalence at secondary school and their feeder primaries for boys and girls separately was r = 0.72 (d.f. 22; P < 0"0001). Discussion
The smoking prevalence estimates of this study are in broad agreement with the O.P.C.S. national study, however they also bear out the suggestion made in that study that regional differences do exist, for example the higher prevalence of regular smoking in the older boys in the North. The findings in the "ever smoked" category are perhaps of even greater relevance in determining the timing of anti-smoking education since, as was stated in the introduction, too early a beginning might even encourage experimentation with cigarettes whilst too late a start with intervention could be ineffective. In considering the findings both on a regional and school basis "ever smoking" and regular smoking patterns must be viewed in conjunction. By the age of 10 or 11 years, the regional results showed that about a quarter had tried smoking but a very small proportion had yet become established smokers which could suggest that this is the approximate time to introduce anti-smoking education. Differences between individual schools, however, suggest that very varied patterns exist. Health authority regions and districts developing policies for anti-smoking education in schools need to produce a scheme which is flexible enough for individual schools to develop their own timing and modifications to suit their own circumstances. It is, however, important to bear in mind that, although such wide variation in smoking prevalence between secondary schools was found, the level of smoking in the Scottish secondary schools showed substantial positive correlation with that of their feeder primary schools. This result, further reinforced by the finding that the widest variations between the northern secondary schools occurred in the first forms, suggests that the level of smoking in a secondary school may not necessarily be a direct consequence of action taken in that school, but of previously existing background factors. Evidence from research in other educational areas, such as academiG achievement, suggests that the effect of schools might perhaps be relatively small. For example, Ainsworth and Batten 6 suggested that "achievement in the secondary school was overwhelmingly
Smoking among British Schoolchildren
249
dependent on what had happened to the pupils before e n t r y " (p. 131). Gray 7 further emphasized how variations between schools might result from their intake. No child is an empty vessel. Any education, including teaching about smoking, will be assimilated or rejected to the extent that the existing beliefs and knowledge allow. Rutter et al. s make the point that it may be that the variations in home background are much greater than those of schools. Remedial action taken by schools might therefore substantially reduce differences in smoking prevalence between schools though it may, as Rutter et al. have suggested in regard to scholastic measures, still be worthwhile in improving the general level in all schools. As Coleman 9 suggested" the increment of achievement that a school provides should be the measure of the school's quality". Thus the aim of smoking education in schools would be to try to reduce smoking prevalence below what it might have been without such education, bearing in mind that there are limits to what can be achieved within schools. There would appear to be a need to survey by brief questionnaire the age ranges 9-12 years in any school system to determine the age at which the first smoking education should be undertaken. Overall national data on smoking prevalence are not "sensitive" enough to answer the question for any one school as to the year-group in which anti-smoking education should be introduced.
Acknowledgements Our particular thanks are due to the Cancer Research Campaign for funding the North of England study; Mr R. L. Davison, Executive Director, Manchester Regional Committee for Cancer Education (North of England); Mr Brian Wilcox, Chief Adviser, MDC Education Department (Sheffield); Mr Donald Reid and the Health Education Council for funding the Sheffield study; and the Department of Community Medicine, Edinburgh University and Scottish ASH (Scotland) and to all the staff and children of the many schools involved.
References 1. Dobbs, J. & Marsh, A. (1983). Smoking Among Secondary School Children: An OPCS Enquiry for the DHSS. London: H.M.S.O. 2. Bartlett, E.E. (1981). The contribution of school health education to community health promotion. What can we reasonably expect? American Journal of Public Health 71, 1384-91. 3. Jeffreys, M. & Westaway, W. R. (1961). Catch them before they start. A report on an attempt to influence children's smoking habits. Health Education Journal 19, 3-17. 4. Jason, L.A., Mollica, M. & Ferrone, L. (1982). Evaluating an early secondary smoking prevention intervention. Preventive Medicine 11, 96-102. 5. McKennell, A. C. (1980). Bias in the reported incidence of smoking by children. International Journal of Epidemiology 9, 167-77. 6. Ainsworth, M. & Batten, E. (1974). The Effects of Environmental Factors on Secondary Education Attainment in Manchester: a Plowden follow-up. London: Macmillan. 7. Gray, J. (1981). Towards effective schools: problems and progress in British research. British Educational Research Journal 7, 59-69. 8. Rutter, M., Maugham, B., Mortimore, P. & Ouston, J., with Smith, A. (1979). Fifteen Thousand Hours: Secondary Schools and Their Effects on Children. London: Open Books. 9. Coleman, J. S. (1975). Methods and results in the IEA studies of effects of school on learning. Review of Educational Research 45, 335-86.
Variations between Schools and Regions in Smoking Prevalence among British Schoolchildren- Implications for Health Education Anne Charlton Cancer ResearchCampaign Fellow
Manchester Regional Committee for Cancer Education Kinnaird Road, Manchester M20 9QL Pam Gillies Lecturer
Department of Community Health, University of Nottingham and Frank L e d w i t h Research Fellow
Department of Education, University of Manchester
Surveys of adolescents' smoking habits in three regions, the north of England, Sheffield and the east of Scotland were conducted concurrently with a national survey (O.P.C.S.) in late autumn 1982. Considerable variations were found in smoking prevalence between regions and also between individual schools. Since the timing of school education about smoking has been found to be critical, and the conclusion drawn suggests that the national data on smoking prevalence is not a sufficiently precise guide, each school may need to conduct its own survey to estimate numbers of children in each age group who have tried smoking or have already become regular smokers. Introduction A recent government national survey 1 has highlighted the extent of the smoking problem among adolescents, A quarter of all secondary schoolchildren are regular smokers by the time they leave school, and adolescent smokers as a whole are smoking an estimated 18 to 24 million cigarettes per week in England and Wales and 2.2-2.8 million in Scotland, on which they are spending well over a million pounds. Within the field of preventive medicine there can therefore be no doubt that anti-smoking education still has a high priority. The timing of school health education about smoking appears to be critical. Too early an introduction m a y lead to increased experimentation as has been shown with other drug education, ~ whilst school interventions have been found to have less impact on those who are already smoking regularly, a, 4 These findings have suggested that it may be important to intervene before the behaviour has become habitual, but not until some of the children have begun to experiment with smoking. In order to do this we need to know when this stage is reached. The findings of the national survey of the Office of Population Censuses and Surveys (O.P.C,S.) suggested that regional and school 0033-3506/85/040243 +07 $02.00/0
© 1985 The Society of Community Medicine
244
A. Charlton et al.
differences could exist. However the report of that survey noted that the study's sampling frame (with small random samples from within individual schools) was designed to estimate national prevalence data but could not be used to estimate differences between schools and could only give a broad indication of regional differences. The surveys described in this report were intended to provide large enough samples, surveyed by standardized methods, to enable regions and individual schools to be compared. Methods
The three surveys covered the north of England: all Cumbria and Tyne and Wear education authorities (65 schools and colleges of further education, sample size: 15,709); Sheffield (10 schools, sample size, 4398); and Scotland (Lothian, Tayside and Grampian education authorities, 13 groups, i.e. a secondary school and its feeder primary schools, sample size, 4897). Although the northern sample included two fee-paying schools, only state primary, middle and secondary schools were included in the analysis described in this paper. Random samples of schools were drawn to provide 10 ~ and 5 ~ samples of children on school rolls in the North and Sheffield respectively. In Scotland a probability sample of secondary schools was drawn within each local authority area. For each high school selected a probability sample was drawn of two feeder primary schools. In the North and Sheffield the sampling frame included all children aged 9 years and upwards in the selected schools, whilst in Scotland only the children in the last year of primary and first year of secondary schools were tested. In all three regions all children in the target year groupings were given the questionnaire. No attempt was made to follow up absentees. For the purposes of this paper, only the 9-16 years age group is described in order to maintain uniformity since students aged 16 years and over may diverge from patterns of full-time compulsory education. Every effort was made to minimize under-reporting of smoking. The questionnaires were anonymous and administered to intact classes which has been found to give the highest reported prevalence. 5 To protect anonymity and thus encourage truthful answers in a test supervised by the class teacher, an envelope was provided into which each individual pupil sealed his or her script on completion. The basic question on smoking behaviour was the same as that used in the O.P.C.S. national survey and asked the children to place themselves within one of six categories ranging from " I have never smoked" to " I usually smoke more than six cigarettes per week". In addition to describing the regular smoker by the standard definition of those who smoked at least one cigarette per week, a n " ever smoked" category was used which included both the current smokers, those who used to smoke and those who had experimented with smoking. The "ever smoked" category may be the more useful one for purposes of deciding the age group at which to introduce anti-smoking education within schools. The surveys were carried out in the late autumn 1982 at the same time as the national survey. The results were analysed for all three regions, using the Statistical Package of the Social Sciences (SPSS) and by Generalized Linear Interactive Modelling (GLIM).
245
Smoking among British Schoolchildren
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A. Charlton et al.
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Smoking among British Schoolchildren
247
TABLE3. Percentages of children who had ever smoked a cigarette in state secondary schools with highest and lowest smoking prevalence in each survey Secondary School Year Groups 1 (Eng.) 2 (Eng.); 1(Scot.) 11-12 years 12-13 years School average Boys
3 (Eng.) 13-14 years
4 (Eng.) 14-15 years
5 (Eng.) 15-16 years
Girls
B o y s Girls
B o y s Girls
B o y s Girls
B o y s Girls
North "Highest school" Base "Lowest school" Base Sheffield "Highest school" % Base "Lowest school" Base Scotland "Highest school" % Base "Lowest school" Base
67 (273)
58 (31)
52 (25)
66 (21)
50 (30)
77 (26)
82 (22)
75 (32)
65 (26)
74 (35)
68 (25)
50 (554)
20 (59)
19 (54)
49 (65)
40 (43)
51 (53)
50 (40)
59 (54)
65 (63)
70 (63)
63 (60)
54 (845)
42 (96)
35 (89)
46 (87)
27 (78)
61 (93)
67 (97)
68 (86)
67 (85)
68 (80)
59 (54)
47 (674)
42 (69)
21 (71)
45 (76)
32 (59)
45 (74)
54 (55)
65 (84)
62 (71)
58 (52)
48 (63)
57 (185)
---
---
63 (102)
51 (83)
. .
. .
. .
. .
. .
. .
30 (139)
---
---
34 (71)
27 (68)
. .
. .
. .
. .
. .
. .
Results 1. Regional differences
Prevalence of regular smoking A l t h o u g h the increasing trend in prevalence of s m o k i n g with increasing age was similar in all three regions, differences were observed between regions at specific ages, as shown in T a b l e 1. Especially striking was the greater prevalence o f s m o k i n g a m o n g s t boys aged 14-15 years (X2 = 14-55; d.f. 1; P < 0.001) a n d boys 15-16 years (Xz = 4.26; d.f. 1 ; P < 0.05) in the N o r t h c o m p a r e d with those in the Sheffield sample. The O.P.C.S. n a t i o n a l survey findings for the c o u n t r y as a whole resembled those of the N o r t h m o r e closely t h a n those of Sheffield. The suggestion m a d e in the n a t i o n a l survey report that regional differences could exist w o u l d therefore seem to be b o r n e out by these findings.
Prevalence o f " ever smoked" As in the case of regular smoking, t h e " ever s m o k e d " category increased in prevalence with increasing age, b u t obviously at a generally higher level (Table 2). T h e r e were, however, few differences between regions in this category, the only statistically significant one occurring in the oldest age group, 15-16 years, as follows (boys, X2 = 5.70; d.f. 1 ; P < 0.02: girls Xz = 19.13; d.f. 1; P < 0-0001).
248
A. Charlton et al.
By the age of 11-12 years, a quarter of the children had at least tried a cigarette once and by the age of 15-16, almost three-quarters of the children in the North and approximately two-thirds in Sheffield had tried smoking. 2. School differences
Although the overall prevalence for "ever smoked" was basically similar for all regions, very considerable differences were observed between individual schools (X2 = 179.47; d.f. 15; P < 0.001 when the standard Chi-Square test was used on all the secondary schools in which first to fifth years were included in the survey, i.e. in the North and Sheffield). GLIM analyses also showed significant differences between schools in each region. In the North, as Table 3 shows, the widest variations between schools lay in the youngest age groups with the gap narrowed with increasing age, which might perhaps suggest a possible "carry-over" from primary schools which diminishes as the children become established at secondary school. However this effect was not observed in Sheffield schools, although it was perhaps modified to some extent by the "middle school" system. In the Scottish schools there was a clear carry-over effect; the Pearson correlation between smoking prevalence at secondary school and their feeder primaries for boys and girls separately was r = 0.72 (d.f. 22; P < 0"0001). Discussion
The smoking prevalence estimates of this study are in broad agreement with the O.P.C.S. national study, however they also bear out the suggestion made in that study that regional differences do exist, for example the higher prevalence of regular smoking in the older boys in the North. The findings in the "ever smoked" category are perhaps of even greater relevance in determining the timing of anti-smoking education since, as was stated in the introduction, too early a beginning might even encourage experimentation with cigarettes whilst too late a start with intervention could be ineffective. In considering the findings both on a regional and school basis "ever smoking" and regular smoking patterns must be viewed in conjunction. By the age of 10 or 11 years, the regional results showed that about a quarter had tried smoking but a very small proportion had yet become established smokers which could suggest that this is the approximate time to introduce anti-smoking education. Differences between individual schools, however, suggest that very varied patterns exist. Health authority regions and districts developing policies for anti-smoking education in schools need to produce a scheme which is flexible enough for individual schools to develop their own timing and modifications to suit their own circumstances. It is, however, important to bear in mind that, although such wide variation in smoking prevalence between secondary schools was found, the level of smoking in the Scottish secondary schools showed substantial positive correlation with that of their feeder primary schools. This result, further reinforced by the finding that the widest variations between the northern secondary schools occurred in the first forms, suggests that the level of smoking in a secondary school may not necessarily be a direct consequence of action taken in that school, but of previously existing background factors. Evidence from research in other educational areas, such as academiG achievement, suggests that the effect of schools might perhaps be relatively small. For example, Ainsworth and Batten 6 suggested that "achievement in the secondary school was overwhelmingly
Smoking among British Schoolchildren
249
dependent on what had happened to the pupils before e n t r y " (p. 131). Gray 7 further emphasized how variations between schools might result from their intake. No child is an empty vessel. Any education, including teaching about smoking, will be assimilated or rejected to the extent that the existing beliefs and knowledge allow. Rutter et al. s make the point that it may be that the variations in home background are much greater than those of schools. Remedial action taken by schools might therefore substantially reduce differences in smoking prevalence between schools though it may, as Rutter et al. have suggested in regard to scholastic measures, still be worthwhile in improving the general level in all schools. As Coleman 9 suggested" the increment of achievement that a school provides should be the measure of the school's quality". Thus the aim of smoking education in schools would be to try to reduce smoking prevalence below what it might have been without such education, bearing in mind that there are limits to what can be achieved within schools. There would appear to be a need to survey by brief questionnaire the age ranges 9-12 years in any school system to determine the age at which the first smoking education should be undertaken. Overall national data on smoking prevalence are not "sensitive" enough to answer the question for any one school as to the year-group in which anti-smoking education should be introduced.
Acknowledgements Our particular thanks are due to the Cancer Research Campaign for funding the North of England study; Mr R. L. Davison, Executive Director, Manchester Regional Committee for Cancer Education (North of England); Mr Brian Wilcox, Chief Adviser, MDC Education Department (Sheffield); Mr Donald Reid and the Health Education Council for funding the Sheffield study; and the Department of Community Medicine, Edinburgh University and Scottish ASH (Scotland) and to all the staff and children of the many schools involved.
References 1. Dobbs, J. & Marsh, A. (1983). Smoking Among Secondary School Children: An OPCS Enquiry for the DHSS. London: H.M.S.O. 2. Bartlett, E.E. (1981). The contribution of school health education to community health promotion. What can we reasonably expect? American Journal of Public Health 71, 1384-91. 3. Jeffreys, M. & Westaway, W. R. (1961). Catch them before they start. A report on an attempt to influence children's smoking habits. Health Education Journal 19, 3-17. 4. Jason, L.A., Mollica, M. & Ferrone, L. (1982). Evaluating an early secondary smoking prevention intervention. Preventive Medicine 11, 96-102. 5. McKennell, A. C. (1980). Bias in the reported incidence of smoking by children. International Journal of Epidemiology 9, 167-77. 6. Ainsworth, M. & Batten, E. (1974). The Effects of Environmental Factors on Secondary Education Attainment in Manchester: a Plowden follow-up. London: Macmillan. 7. Gray, J. (1981). Towards effective schools: problems and progress in British research. British Educational Research Journal 7, 59-69. 8. Rutter, M., Maugham, B., Mortimore, P. & Ouston, J., with Smith, A. (1979). Fifteen Thousand Hours: Secondary Schools and Their Effects on Children. London: Open Books. 9. Coleman, J. S. (1975). Methods and results in the IEA studies of effects of school on learning. Review of Educational Research 45, 335-86.