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Further validation of a questionnaire method for classifying depressive illness
Journal of Affective Disorders, 1 (1979) 179-185 0 Elsevier/North-Holland Biomedical Press
179
FURTHER VALIDATION OF A QUESTIONNAIRE CLASSIFYING DEPRESSIVE ILLNESS
METHOD
FOR
I. PILOWSKY Department of Psychiatry, The Adelaide, S.A. 5000 (Australia)
____
University
of Adelaide,
Royal
_~___
Adelaide
Hospital,
__~_
SUMMARY A total of 367 patients admitted to a psychiatric facility completed the LPD questionnaire. By application of a decision rule to their responses, they were classified as ‘nondepression’ and ‘non-depressive syndrome’. endogenous depression ’ , ‘endogenous This classification was found to be associated significantly with their categorization on the basis of clinical diagnosis. The findings suggest that this method of classification on the basis of responses to the LPD may have a useful research and clinical role.
INTRODUCTION
In previous papers (Pilowsky et al. 1969; Pilowsky and Boulton 1970; Pilowsky and McGrath 1970; Pilowsky and Spalding 1972) the development of a depression questionnaire and its use in the identification and classification of depressed patients, has been described. The first of these (Pilowsky et al. 1969) described the classification of 200 patients on the basis of their responses to the Levine-Pilowsky Depression questionnaire (LPD) and also certain other attributes (age, marital status, sex and length of illness). Using a form of numerical taxonomy based on information theory (Wallace and Boulton 1968) the subjects were grouped into 3 classes which were described on the basis of the LPD items which discriminated between them. Thus Class A was considered to represent a mixed group of depressive reactions; Class B corresponded very strikingly to ‘endogenous’ depression; and Class C comprised subjects with a nondepressive syndrome. When the relationship between clinical diagnosis and taxonomic classification was examined, a significant association was found, of which the most striking feature was a preponderance of patients clinically diagnosed as ‘endogenous’ depression in Class By while nondepressive svndromes in this class, ~__ ~~~__~~~~ _~~~_ mum-- ~~~~~-were underreoresented On the basis of these findings it was concluded that endogenous depression is a syndrome which provides a useful basis for classifying depressed patients
180
and is unlikely to be a consequence of observer bias. Following this application of numerical taxonomy to the problem of classifying depressive illnesses, Pilowsky and Boulton (1970) developed a decision rule for identifying class members. The assignment of patients to Class A or Class B was then related to their likelihood of treatment with ECT and to their response to such therapy. It was found that Class B patients were more likely to be treated with ECT, and furthermore showed a better response than those Class A patients who were treated in this way. In a later study, it was also found that responses to items contributing to a high Class B score were more likely to change after ECT (Pilowsky and McGrath 1970). In addition, Byrne (1975) found a highly significant association between clinical diagnosis and classification; as well as between classification and G.S.R. inhibition threshold. He also found that the LPD depression score (Pilowsky and Spalding 1972) correlated very highly with the Zung depression score, r = 0.88. The chief purpose of this study has been to examine the relationship between classification using the LPD questionnaire and clinical diagnosis in a heterogeneous population of psychiatric patients. It will also be apparent that while the decision rule has been used to provide a categorisation of patients, it does in fact also provide a score which reflects the probability of class membership. In the present study the relationship of this ‘score’ to diagnosis has also been examined. SUBJECTS
AND METHODS
The subjects for this study were consecutive inpatients of the pscyhiatric ward of the Royal Adelaide Hospital, South Australia, who were administered the questionnaire within one week of admission to the ward. Patients not included were the excessively uncooperative, those with intellectual impairment, and those whose clinical diagnoses indicated organic cerebral involvement. Testing was individual, and Greek and Italian translations of the questionnaire were available. The final population was comprised of 367 patients (mean age 38.9, SD 16.4). Of these, 123 were male (mean age 40.8, SD 17.3) and 244 were female (mean age 37.9, SD 15.8). Each patient’s clinical diagnosis was classified into 1 of 5 categories, as follows: depressive neurosis (non-endogenous depression) (n = 114), endogenous depression (n = 99), nondepressive neurosis (n = 24), non-depressive psychosis (n = 50) and other (n = 80). The non-depressive neurosis category included those patients with anxiety states and traumatic neurosis, while the nondepressive psychosis category included mainly patients with schizophrenic illnesses, as well as those in the manic phase of a manic-depressive illness. The category labelled ‘other’ included all those patients whose diagnoses (e.g. personality disorder, anorexia nervosa) did not conform to the criteria of any of the other categories. For the purposes of analysis, the last
181
three categories were subsumed under one heading: ‘no depressive syndrome’. The categorisation of patients in this way was only done after all the questionnaire data had been collected. The diagnosis used for each patient was that made by the attending psychiatrist on or about the date that the questionnaire was filled in. RESULTS
(a) Depression score The mean depression score obtained by patients in each diagnostic category is shown in Table 1. An analysis of variance showed that the differences between categories were significant at the P < 0.001 level. Mean depression scale scores of both nonendogenous and endogenous depression groups were significantly higher than that of the control group (t = 2.86, df = 266, P < 0.005; and t = 3.90, respectively). There was no significant difference df = 251, P< 0.001, between the two depressed groups (t = -0.94, df = 211, P > 0.1). Males on the whole were found to score lower than females (P < 0.05). However, even when this effect was controlled, the diagnostic category remained the effective influence on the depression score of the subject (P< 0.001). The effect of the subject’s age on his or her score was also analysed, and no influence was revealed (f = 0.885, df = 61, P > 0.1). (b) Taxonomic classification Taxonomic grouping of the subjects according to the decision rule described by Pilowsky and Boulton (1970) led to 75 patients being classified as Class A (non-endogenous depression), 135 as Class B (endogenous depression) and 157 as Class C (no depressive syndrome). Of the males, 26 had nonendogenous depression, 35 endogenous depression, and 62 had no TABLE MEAN
1 DEPRESSION
SCORES
Diagnosis Non-endogenous (n = 114) Endogenous (n = 99) No depressive (n = 154)
depression
depression
syndrome
Population
Mean
SD
Total Male Female
9.89 10.03 9.85
4.33 3.01 4.71
Total Male Female
10.41 9.78 10.79
3.63 3.23 3.83
8.36 6.80 9.29
4.34 4.10 4.22
Total Male Female
182 TABLE
2
RELATIONSHIP CLASSIFICATIONS
BETWEEN
Diagnosis
n
CLINICAL
Endogenous depression No depressive syndrome Total
AND
LPD
DECISION
RULE
Classification Non-endogenous depression
Non-endogenous depression
DIAGNOSIS
Endogenous depression
No depressive syndrome
% identified by LPD
114
38
42
34
33.3
99
9
58
32
58.6
154
28
35
YI
59.1
367
75
135
157
51.0
depressive syndrome. Of the females, 49 fell in Class A, 100 in Class B and 95 in Class C. There was no significant difference between males and females in terms of their taxonomic classification. The clinical diagnosis of the patient was then related to the taxonomic category to which he or she had been assigned by the LPD questionnaire decision rule (Table 2). A highly significant association emerged (chisquare = 53.5, df = 4, P < 0.001). Since the age of the subject is taken into account as part of the decision rule, it seemed of interest to discover whether this was a major contributor to the relationship between the two classifications by repeating the analysis after applying the decision rule with a constant loading for age in all cases. This demonstrated a highly significant association between the clinical diagnostic categories and the taxonomic classes independent of age (x2 = 47.65, df = 4, P< 0.001). (c) Application of the decision rule As previously mentioned, the ‘score’ produced by the LPD decision rule was also examined in the present study. The relationship between this ‘A vs B’ score and the clinical diagnostic category of the patient is shown in Table 3. An analysis of variance revealed a significant difference between diagnostic categories (f = 12.085, df = 2, P < 0.001). The sex of the subject did affect the resulting ‘A vs B’ score to a limited extent (P < 0.05), females in each category scoring more towards the ‘B’ or ‘endogenous’ pole than the males. When a two-way analysis of variance was performed which controlled for sex effects, the diagnostic category of the patient retained its influence over the obtained ‘A vs B’ score (f = 19.974, df = 2,P< 0.001). As noted earlier in relation to the results of the taxonomic classification,
183 TABLE
3
MEAN SCORES OBTAINED BY APPLICATION TION TO CLINICAL DIAGNOSIS Clinical
diagnostic
Non-endogenous
Endogenous
No depressive
TABLE
category depression
depression
OF THE
RULE
Population
Mean
SD
Total Male Female
1.44 2.16 1.19
6.00 5.98 6.01
Total Male Female
syndrome
DECISION
-2.42 -2.05 -2.65
Total Male Female
2.40 4.04 1.44
IN RELA-
6.18 6.16 6.23 6.30 5.66 6.48
4
COMPARISON TIC GROUPS
OF MEAN
VALUES
OF “A
VS B” SCORES
Endogenous Non-endogenous depression Endogenous depression
t = -1.80
depression a
OF CLINICAL
No depressive
DIAGNOS-
syndrome
t = 0.79 t = 8.95 a
a P < 0.001.
the age of the patient is used in determining the ‘A vs B’ score by use of the decision rule. In order to determine whether age is the only factor that plays a significant role in the association between the decision rule score and clinical diagnosis, an analysis of variance was performed with age as a covariate. It was found that the clinical category of the patient retained a significant effect (f = 5.956, df = 2, P < 0.005). Mean decision rule scores attained by patients in the three clinical categories were compared and the resultant t-values are presented in Table 4. DISCUSSION
The decision rule applied to the Levine-Pilowsky depression questionnaire identifies patients as being members of one of three depression-related categories. In doing so, it provides information as to the similarity between the response pattern of an individual patient and the typical response pattern of the patients comprising the original taxonomically generated groups. Thus it carries out a procedure essentially the same as that which a clinician undertakes whenever he compares a patient’s presentation to that of those he has
184
previously seen and diagnosed. In this sense the exercise is one of pattern recognition. This rather complex process can obviously never be executed to the point of therapeutic decision-making on the basis of questionnaire responses alone. Nevertheless it is a process whose objectivity and reproducibility may be improved in a number of ways by such instruments, as well as the use of rating scales, multiple observers and standardized interviews. The LPD thus falls into the category of such acids to objectivity, and there seems little doubt that a need for such instruments exists in the area of depressive illness. In this regard it is interesting to note that the LPD classification achieves greatest agreement with clinical diagnosis in the categories of ‘endogenous depression’ and ‘nondepressive syndrome’, and least agreement in the category of ‘non-endogenous (or neurotic) depression’. This is in keeping with the findings of Kreitman et al. (1961), that psychiatrists attained high (but not complete) agreement as to the presence of depression but had marked difficulty agreeing as to whether a condition was ‘endogenous’ or ‘neurotic’. In general, least diagnostic agreement was achieved in the neurotic disorders (as opposed to psychotic and organic conditions). Thus the relationship found between LPD ‘diagnosis’ and clinical diagnosis is similar in some respects to that which might be found between two psychiatrists using similar diagnostic categories but without prior detailed agreement as to diagnostic criteria. A further conclusion which can be drawn from this study is that the grouping generated by the questionnaire study of the original 200 English patients (Pilowsky et al. 1969) may be regarded as having validity for other populations. Thus it offers support for the original descriptions of Class A as nonendogenous depression, Class B as endogenous depression and Class C as non-depressive. The advantage of a questionnaire method for classifying depressive syndromes in this way obviously lies in its relative economy and simplicity of administration, as well as the possibilities it offers for comparing patients in different centres. At the same time, it must be recognised that the LPD decision rule is not sensitive to the other subtypes of depression which have been described (Paykel 1971). However, isofar as ‘endogenous depression’ is well recognised as a clinically significant entity it seems worthwhile to be able to quantify this aspect of depression, in as objective a manner as possible. Whether or not the LPD proves a truly useful instrument in this regard will depend to a considerable extent on the ability of other workers to replicate our findings. Thus far the evidence suggests that the LPD decision rule classifies psychiatric patients in a clinically meaningful and useful way. ACKNOWLEDGEMENTS
I wish to acknowledge Amanda Gordon.
the helpful
assistance
given by Neil Spence
and
185 REFERENCES Byrne, D.G., Some preliminary observations on a questionnaire technique for classifying depressive illness - Its relationship with clinical diagnosis and a biological technique for depressive classification, Aust. N.Z. J. Psychiat., 9 (1975) 25-29. Kreitman, N., Sainsbury, P., Morrissey, J., Towers, J. and Scriviner, J., The reliability of psychiatric assessment -An analysis, J. Ment. Sci., 107 (1961) 887-908. Paykel, E.S., Classification of depressed patients - A cluster analysis derived grouping, Brit. J. Psychiat., 118 (1971) 275-288. Pilowsky, I. and Boulton, D.M., Development of a questionnaire-based decision rule for classifying depressed patients, Brit. J. Psychiat., 16 (1970) 647-650. Pilowsky, I. and McGrath, M.D., The effect of electro-convulsive therapy on responses to a depression questionnaire -Implications for taxonomy, Brit. J. Psychiat., 117 (1970) 685-688. Pilowsky, I. and Spalding, D., A method of measuring depression, Brit. J. Psychiat., 121 (1972) 411-416. Pilowsky, I., Levine, S. and Boulton, D.M., The classification of depression by numerical taxonomy, Brit. J. Psychiat., 115 (1969) 937-945. Wallace, C.S. and Boulton, D.M., An information measure for classification, Computer J., 11 (1968) 185-194.
179
FURTHER VALIDATION OF A QUESTIONNAIRE CLASSIFYING DEPRESSIVE ILLNESS
METHOD
FOR
I. PILOWSKY Department of Psychiatry, The Adelaide, S.A. 5000 (Australia)
____
University
of Adelaide,
Royal
_~___
Adelaide
Hospital,
__~_
SUMMARY A total of 367 patients admitted to a psychiatric facility completed the LPD questionnaire. By application of a decision rule to their responses, they were classified as ‘nondepression’ and ‘non-depressive syndrome’. endogenous depression ’ , ‘endogenous This classification was found to be associated significantly with their categorization on the basis of clinical diagnosis. The findings suggest that this method of classification on the basis of responses to the LPD may have a useful research and clinical role.
INTRODUCTION
In previous papers (Pilowsky et al. 1969; Pilowsky and Boulton 1970; Pilowsky and McGrath 1970; Pilowsky and Spalding 1972) the development of a depression questionnaire and its use in the identification and classification of depressed patients, has been described. The first of these (Pilowsky et al. 1969) described the classification of 200 patients on the basis of their responses to the Levine-Pilowsky Depression questionnaire (LPD) and also certain other attributes (age, marital status, sex and length of illness). Using a form of numerical taxonomy based on information theory (Wallace and Boulton 1968) the subjects were grouped into 3 classes which were described on the basis of the LPD items which discriminated between them. Thus Class A was considered to represent a mixed group of depressive reactions; Class B corresponded very strikingly to ‘endogenous’ depression; and Class C comprised subjects with a nondepressive syndrome. When the relationship between clinical diagnosis and taxonomic classification was examined, a significant association was found, of which the most striking feature was a preponderance of patients clinically diagnosed as ‘endogenous’ depression in Class By while nondepressive svndromes in this class, ~__ ~~~__~~~~ _~~~_ mum-- ~~~~~-were underreoresented On the basis of these findings it was concluded that endogenous depression is a syndrome which provides a useful basis for classifying depressed patients
180
and is unlikely to be a consequence of observer bias. Following this application of numerical taxonomy to the problem of classifying depressive illnesses, Pilowsky and Boulton (1970) developed a decision rule for identifying class members. The assignment of patients to Class A or Class B was then related to their likelihood of treatment with ECT and to their response to such therapy. It was found that Class B patients were more likely to be treated with ECT, and furthermore showed a better response than those Class A patients who were treated in this way. In a later study, it was also found that responses to items contributing to a high Class B score were more likely to change after ECT (Pilowsky and McGrath 1970). In addition, Byrne (1975) found a highly significant association between clinical diagnosis and classification; as well as between classification and G.S.R. inhibition threshold. He also found that the LPD depression score (Pilowsky and Spalding 1972) correlated very highly with the Zung depression score, r = 0.88. The chief purpose of this study has been to examine the relationship between classification using the LPD questionnaire and clinical diagnosis in a heterogeneous population of psychiatric patients. It will also be apparent that while the decision rule has been used to provide a categorisation of patients, it does in fact also provide a score which reflects the probability of class membership. In the present study the relationship of this ‘score’ to diagnosis has also been examined. SUBJECTS
AND METHODS
The subjects for this study were consecutive inpatients of the pscyhiatric ward of the Royal Adelaide Hospital, South Australia, who were administered the questionnaire within one week of admission to the ward. Patients not included were the excessively uncooperative, those with intellectual impairment, and those whose clinical diagnoses indicated organic cerebral involvement. Testing was individual, and Greek and Italian translations of the questionnaire were available. The final population was comprised of 367 patients (mean age 38.9, SD 16.4). Of these, 123 were male (mean age 40.8, SD 17.3) and 244 were female (mean age 37.9, SD 15.8). Each patient’s clinical diagnosis was classified into 1 of 5 categories, as follows: depressive neurosis (non-endogenous depression) (n = 114), endogenous depression (n = 99), nondepressive neurosis (n = 24), non-depressive psychosis (n = 50) and other (n = 80). The non-depressive neurosis category included those patients with anxiety states and traumatic neurosis, while the nondepressive psychosis category included mainly patients with schizophrenic illnesses, as well as those in the manic phase of a manic-depressive illness. The category labelled ‘other’ included all those patients whose diagnoses (e.g. personality disorder, anorexia nervosa) did not conform to the criteria of any of the other categories. For the purposes of analysis, the last
181
three categories were subsumed under one heading: ‘no depressive syndrome’. The categorisation of patients in this way was only done after all the questionnaire data had been collected. The diagnosis used for each patient was that made by the attending psychiatrist on or about the date that the questionnaire was filled in. RESULTS
(a) Depression score The mean depression score obtained by patients in each diagnostic category is shown in Table 1. An analysis of variance showed that the differences between categories were significant at the P < 0.001 level. Mean depression scale scores of both nonendogenous and endogenous depression groups were significantly higher than that of the control group (t = 2.86, df = 266, P < 0.005; and t = 3.90, respectively). There was no significant difference df = 251, P< 0.001, between the two depressed groups (t = -0.94, df = 211, P > 0.1). Males on the whole were found to score lower than females (P < 0.05). However, even when this effect was controlled, the diagnostic category remained the effective influence on the depression score of the subject (P< 0.001). The effect of the subject’s age on his or her score was also analysed, and no influence was revealed (f = 0.885, df = 61, P > 0.1). (b) Taxonomic classification Taxonomic grouping of the subjects according to the decision rule described by Pilowsky and Boulton (1970) led to 75 patients being classified as Class A (non-endogenous depression), 135 as Class B (endogenous depression) and 157 as Class C (no depressive syndrome). Of the males, 26 had nonendogenous depression, 35 endogenous depression, and 62 had no TABLE MEAN
1 DEPRESSION
SCORES
Diagnosis Non-endogenous (n = 114) Endogenous (n = 99) No depressive (n = 154)
depression
depression
syndrome
Population
Mean
SD
Total Male Female
9.89 10.03 9.85
4.33 3.01 4.71
Total Male Female
10.41 9.78 10.79
3.63 3.23 3.83
8.36 6.80 9.29
4.34 4.10 4.22
Total Male Female
182 TABLE
2
RELATIONSHIP CLASSIFICATIONS
BETWEEN
Diagnosis
n
CLINICAL
Endogenous depression No depressive syndrome Total
AND
LPD
DECISION
RULE
Classification Non-endogenous depression
Non-endogenous depression
DIAGNOSIS
Endogenous depression
No depressive syndrome
% identified by LPD
114
38
42
34
33.3
99
9
58
32
58.6
154
28
35
YI
59.1
367
75
135
157
51.0
depressive syndrome. Of the females, 49 fell in Class A, 100 in Class B and 95 in Class C. There was no significant difference between males and females in terms of their taxonomic classification. The clinical diagnosis of the patient was then related to the taxonomic category to which he or she had been assigned by the LPD questionnaire decision rule (Table 2). A highly significant association emerged (chisquare = 53.5, df = 4, P < 0.001). Since the age of the subject is taken into account as part of the decision rule, it seemed of interest to discover whether this was a major contributor to the relationship between the two classifications by repeating the analysis after applying the decision rule with a constant loading for age in all cases. This demonstrated a highly significant association between the clinical diagnostic categories and the taxonomic classes independent of age (x2 = 47.65, df = 4, P< 0.001). (c) Application of the decision rule As previously mentioned, the ‘score’ produced by the LPD decision rule was also examined in the present study. The relationship between this ‘A vs B’ score and the clinical diagnostic category of the patient is shown in Table 3. An analysis of variance revealed a significant difference between diagnostic categories (f = 12.085, df = 2, P < 0.001). The sex of the subject did affect the resulting ‘A vs B’ score to a limited extent (P < 0.05), females in each category scoring more towards the ‘B’ or ‘endogenous’ pole than the males. When a two-way analysis of variance was performed which controlled for sex effects, the diagnostic category of the patient retained its influence over the obtained ‘A vs B’ score (f = 19.974, df = 2,P< 0.001). As noted earlier in relation to the results of the taxonomic classification,
183 TABLE
3
MEAN SCORES OBTAINED BY APPLICATION TION TO CLINICAL DIAGNOSIS Clinical
diagnostic
Non-endogenous
Endogenous
No depressive
TABLE
category depression
depression
OF THE
RULE
Population
Mean
SD
Total Male Female
1.44 2.16 1.19
6.00 5.98 6.01
Total Male Female
syndrome
DECISION
-2.42 -2.05 -2.65
Total Male Female
2.40 4.04 1.44
IN RELA-
6.18 6.16 6.23 6.30 5.66 6.48
4
COMPARISON TIC GROUPS
OF MEAN
VALUES
OF “A
VS B” SCORES
Endogenous Non-endogenous depression Endogenous depression
t = -1.80
depression a
OF CLINICAL
No depressive
DIAGNOS-
syndrome
t = 0.79 t = 8.95 a
a P < 0.001.
the age of the patient is used in determining the ‘A vs B’ score by use of the decision rule. In order to determine whether age is the only factor that plays a significant role in the association between the decision rule score and clinical diagnosis, an analysis of variance was performed with age as a covariate. It was found that the clinical category of the patient retained a significant effect (f = 5.956, df = 2, P < 0.005). Mean decision rule scores attained by patients in the three clinical categories were compared and the resultant t-values are presented in Table 4. DISCUSSION
The decision rule applied to the Levine-Pilowsky depression questionnaire identifies patients as being members of one of three depression-related categories. In doing so, it provides information as to the similarity between the response pattern of an individual patient and the typical response pattern of the patients comprising the original taxonomically generated groups. Thus it carries out a procedure essentially the same as that which a clinician undertakes whenever he compares a patient’s presentation to that of those he has
184
previously seen and diagnosed. In this sense the exercise is one of pattern recognition. This rather complex process can obviously never be executed to the point of therapeutic decision-making on the basis of questionnaire responses alone. Nevertheless it is a process whose objectivity and reproducibility may be improved in a number of ways by such instruments, as well as the use of rating scales, multiple observers and standardized interviews. The LPD thus falls into the category of such acids to objectivity, and there seems little doubt that a need for such instruments exists in the area of depressive illness. In this regard it is interesting to note that the LPD classification achieves greatest agreement with clinical diagnosis in the categories of ‘endogenous depression’ and ‘nondepressive syndrome’, and least agreement in the category of ‘non-endogenous (or neurotic) depression’. This is in keeping with the findings of Kreitman et al. (1961), that psychiatrists attained high (but not complete) agreement as to the presence of depression but had marked difficulty agreeing as to whether a condition was ‘endogenous’ or ‘neurotic’. In general, least diagnostic agreement was achieved in the neurotic disorders (as opposed to psychotic and organic conditions). Thus the relationship found between LPD ‘diagnosis’ and clinical diagnosis is similar in some respects to that which might be found between two psychiatrists using similar diagnostic categories but without prior detailed agreement as to diagnostic criteria. A further conclusion which can be drawn from this study is that the grouping generated by the questionnaire study of the original 200 English patients (Pilowsky et al. 1969) may be regarded as having validity for other populations. Thus it offers support for the original descriptions of Class A as nonendogenous depression, Class B as endogenous depression and Class C as non-depressive. The advantage of a questionnaire method for classifying depressive syndromes in this way obviously lies in its relative economy and simplicity of administration, as well as the possibilities it offers for comparing patients in different centres. At the same time, it must be recognised that the LPD decision rule is not sensitive to the other subtypes of depression which have been described (Paykel 1971). However, isofar as ‘endogenous depression’ is well recognised as a clinically significant entity it seems worthwhile to be able to quantify this aspect of depression, in as objective a manner as possible. Whether or not the LPD proves a truly useful instrument in this regard will depend to a considerable extent on the ability of other workers to replicate our findings. Thus far the evidence suggests that the LPD decision rule classifies psychiatric patients in a clinically meaningful and useful way. ACKNOWLEDGEMENTS
I wish to acknowledge Amanda Gordon.
the helpful
assistance
given by Neil Spence
and
185 REFERENCES Byrne, D.G., Some preliminary observations on a questionnaire technique for classifying depressive illness - Its relationship with clinical diagnosis and a biological technique for depressive classification, Aust. N.Z. J. Psychiat., 9 (1975) 25-29. Kreitman, N., Sainsbury, P., Morrissey, J., Towers, J. and Scriviner, J., The reliability of psychiatric assessment -An analysis, J. Ment. Sci., 107 (1961) 887-908. Paykel, E.S., Classification of depressed patients - A cluster analysis derived grouping, Brit. J. Psychiat., 118 (1971) 275-288. Pilowsky, I. and Boulton, D.M., Development of a questionnaire-based decision rule for classifying depressed patients, Brit. J. Psychiat., 16 (1970) 647-650. Pilowsky, I. and McGrath, M.D., The effect of electro-convulsive therapy on responses to a depression questionnaire -Implications for taxonomy, Brit. J. Psychiat., 117 (1970) 685-688. Pilowsky, I. and Spalding, D., A method of measuring depression, Brit. J. Psychiat., 121 (1972) 411-416. Pilowsky, I., Levine, S. and Boulton, D.M., The classification of depression by numerical taxonomy, Brit. J. Psychiat., 115 (1969) 937-945. Wallace, C.S. and Boulton, D.M., An information measure for classification, Computer J., 11 (1968) 185-194.