The internal and external validity of the major depression inventory in measuring severity of depressive states

Psychological Medicine, 2003, 33, 351–356.
The internal and external validity of the Major Depression Inventory in measuring severity of depressive states L. R. O L S E N,1 D. V. J E N S E N, V. N O E R H O L M, K. M A R T I N Y A N D P. B E C H From the Psychiatric Research Unit, Frederiksborg General Hospital, Hillerød ; and Department of Rheumatology, Hoersholm General Hospital, Hoersholm, Denmark Background. We have developed the Major Depression Inventory (MDI), consisting of 10 items,covering the DSM-IV as well as the ICD-10 symptoms of depressive illness. We aimed to evaluate thisas a scale measuring severity of depressive states with reference to both internal and external validity.
Method. Patients representing the score range from no depression to marked depression on theHamilton Depression Scale (HAM-D) completed the MDI. Both classical and modern psychometricmethods were applied for the evaluation of validity, including the Rasch analysis.
Results. In total, 91 patients were included. The results showed that the MDI had an adequateinternal validity in being a unidimensional scale (the total score an appropriate or sufficient statistic).
The external validity of the MDI was also confirmed as the total score of the MDI correlatedsignificantly with the HAM-D (Pearson’s coefficient 0.86, Pf0.01, Spearman 0.80, Pf0.01).
Conclusion. When used in a sample of patients with different states of depression the MDI has anadequate internal and external validity.
questionnaires (BDI, SDS and CES-D) all con-tain around 20 symptoms which, however, have The most frequently used self-rating scales for a limited coverage of the nine DSM-III symp- depression are the Beck Depression Inventory toms of major depression. On this background, (BDI) (Beck et al. 1961), the Zung Self-Rating we developed the Major Depression Inventory Depression Scale (SDS) (Zung, 1965) and the (MDI) (Bech, 1998 ; Bech et al. 2001), which Center for Epidemiological Studies Depression covers the whole spectrum of symptoms in both Scale (CES-D) (Radloff, 1977). These question- the DSM-III/DSM-IV (APA, 1994) ‘ major de- naires have been psychometrically evaluated as pression ’ and the ICD-10 (WHO, 1993) ‘ mod- scales to measure the severity of depressive states and also as screening instruments for the diag- On the basis of the algorithms for diagnosing depression in accordance with DSM-IV or ICD- 10 the MDI showed a high sensitivity and speci- 1980) with symptom-based diagnostic criteria ficity in a previous study (Bech et al. 2001).
for mental disorders, the diagnosis of major de- In the present study we have investigated the pression is reached using an algorithm cover- MDI as a scale for measuring severity of de- ing only nine symptoms. The three depression pressive states. The analysis of the MDI has fo-cused on both the internal validity (i.e. tests for 1 Address for correspondence: Dr Lis Raabaek Olsen, Psychiatric unidimensionality) and the external validity (i.e.
Research Unit, Frederiksborg General Hospital, DK-3400 Hillerød,Denmark.
the correspondence with a clinician rated scale).
item of guilt. Thus, the MDI contains 10 items, however, items 8 and 10 are divided into two sub- have been to evaluate the internal validity of items, a and b (Appendix 1). Only the highest the scale (the total score being an appropriate scores of items 8 and 10 (either a or b) are in- or sufficient statistic) as well as the external cluded in the statistical analysis. On a 6-point validity of the scale (the correlation of the MDI Likert scale, the individual items measure how with the Hamilton Depression Scale (HAM-D (Hamilton, 1967 ; Bech et al. 1986)), which in- present during the past 14 days. The scale goes cludes a standardization of the MDI with cut-off from 0 (the symptom has not been present at all) scores in terms of the HAM-D definitions of mild to 5 (the symptom has been present all of the time). The various steps refer to the frequency ofthe symptoms during the last 2 weeks and aredefined by adverbs or adjectives (Appendix 1) with only indirect definitions. In a previous study (Bent-Hansen et al. 1995) it had been found that The patients were selected from ongoing studies depressed patients prefer such indirect stipu- within the following range of depressive states.
lations to a direct or definite item manual for theindividual items.
These were out-patients from our Department diagnostic instrument with the algorithms lead- of Rheumatology ; we consecutively included ing to the DSM-IV or ICD-10 categories ‘ major ’ patients who had suffered from low back pain or ‘ moderate to severe ’ depression (Bech et al.
for more than 3 months without psychiatric 2001), and as a measuring instrument in which the total score is a sufficient statistic. When usedas a measuring instrument, the 10 items are added up, with a theoretical score range from0 to 50.
These out-patients were from a private psychi-atric practice in Copenhagen and they had been screened for inclusion in a study on social ad-aptation.
We used the 17-item (HAM-D17) version en-dorsed by Max Hamilton and published by Bech et al. (1986). This version has been used in thestudies performed by the Danish University These out-patients from our Psychiatric Re- Antidepressant Group (e.g. DUAG, 1990). The search Unit were participating in an ongoing HAM-D raters who participated in the present study on light therapy in major depression with- study had been trained as investigators in the out SAD (seasonal affective disorder).
DUAG trials. The intraclass coefficients of re- liability in the DUAG trials are 0.75 or higher(Stage et al. 2001).
These in-patients from our Psychiatric HospitalDepartment were participating in a study on the sensitivity and specificity of the MDI, using thePresent State Examination (PSE) as the index of diagnostic validity (Bech et al. 2001).
A factor analysis in terms of a principal com- ponent analysis was performed (Nunnally & Bernstein, 1994). A scree plot was used to de- The items of the scale cover the ten ICD-10 termine the numbers of factors to be taken into symptoms of depression. These symptoms are consideration. A ‘ general factor ’ was defined as a identical with the DSM-IV major depression factor explaining at least 50 % of the variance.
symptoms apart from one symptom, low self- Cronbach’s coefficient alpha was used to evalu- esteem, which in DSM-IV is incorporated in the ate internal consistency. A coefficient of 0.80 or The MDI scale for measuring severity of depression higher was considered adequate (Nunnally & Traditionally, the number of patients needed psy-chometrically when using principal component analysis is approximately 10 times the number ofitems in the scale under examination (Aiken, The Rasch analysis (Rasch, 1960 ; Bech et al.
1995). As the MDI contains ten items, the num- 1981 ; Allerup, 1997) was used to test for uni- ber of patients should be approximately 100.
dimensionality of the scale. The test of fit of theRasch model for the total scale score being asufficient statistic was performed by use of the one parameter logistic programme in which thecriteria of males versus females and patients with In total, 91 patients (24 males, 67 females ; mean low scores versus patients with high scores were age 45.5 years, S.D. 15.2) were included in the tested (Verhelst & Glass, 1995). The non-para- psychometric analysis of the MDI. Of those, 18 metric evaluation of the data structure in ac- patients were recruited from the Department of cordance with the Rasch model was performed Rheumatology (5 males, 13 females, mean age using the Mokken analysis (Mokken, 1971 ; De 43.0 years, S.D. 15.2, HAM-D17 mean score 6.1, Jong & Molenaar, 1987 ; Molenaar et al. 1994).
S.D. 5.9), 11 patients were recruited from our Out- The Mokken analysis of homogeneity or uni- patient Research Unit (4 males, 7 females ; mean dimensionality is a measure of the extent to which age 48.5 years, S.D. 11.1, HAM-D17 mean score an extra item fits into the structure provided 20.6, S.D. 4.6), 40 patients were recruited from by the other items of the scale. The test of fit of the private psychiatric practice (13 males, 27 fe- the individual items analogously to the Mokken males ; mean age 40.6 years, S.D. 15.5, HAM-D17 analysis was within the Rasch analysis per- mean score 18.9, S.D. 7.5), and 22 in-patients formed as described by Allerup (1997). Each item were recruited from our Psychiatric Department was first dichotomized by rescoring grades 0, 1 (2 males, 20 females ; mean age 55.0 years, S.D.
and 2 as 0, and grades 3, 4 and 5 as 1. The level 15.1). In the latter sample of in-patients, all of rejection of unidimensionality in the Rasch patients had a mood disorder, 15 patients had a analysis was Pf0.01. As external criterion the current diagnosis of major depression (HAM- level of acceptance according to the Mokken D17 mean score 21.5, S.D. 5.5) and the remaining analysis was a coefficient of homogeneity of 7 patients had major depression in remission o0.40, while a coefficient of 0.30 to 0.39 was (HAM-D17 mean score 11.1, S.D. 6.3). In the considered only to be just acceptable (Mokken, different groups, the percentage of patients with a HAM-D17 score of o18 ranged from 5.6 % to66.7 %.
The Hamilton Depression Scale (HAM-D) was used as the index of external validity in the 17- item version (HAM-D17). As to the measure of identified only one factor when the scree plot was correlation, the Pearson coefficient is reported in analysed. This factor explained 56 % of the some studies while the Spearman coefficient is variance while the second factor explained 10 %, reported in other studies. Therefore, the strength the third factor 8 % and the fourth factor 5 % of the association between the MDI and the of the variance. Table 1 shows the factor load- HAM-D17 was expressed in terms of the Pearson ings for the individual items according to the prin- coefficient (Altman, 1991) as well as the Spear- cipal component analysis, indicating a higher man coefficient (Siegel, 1956). The association loading in the top-listed items compared to the between the MDI score and the HAM-D score bottom-listed. Cronbach’s coefficient alpha was to standardize the MDI was estimated by linear 0.90. Table 1 shows also the results of the regression analysis in which the MDI score was Mokken analysis with the Loevinger coefficient considered the dependent variable. The stan- of homogeneity for the total scores and for the dardization included prediction intervals of the individual items. Although two of the items had Loevinger coefficients of <0.40 (item 9 and 10) By linear regression in which the MDI score the 10 items of the MDI with the corresponding was considered as the dependent variable the factor loadings from the principal component following equation was estimated (confidence analysis. The items are listed in terms of inclu- siveness (rank-ordered ), i.e. highest mean score for ‘ lack of energy ’ and lowest mean score for ‘ sui- For this estimation the value of R2 is 0.73, i.e. the proportion of the total variation of the depen- dent variable explained by this model is 73 %.
Table 2 shows the standardization of the MDI using the conventional cut-off scores on the HAM-D17 as index of validity (Bech et al. 1975).
The range of scores obtained on the Hamilton population in the present study had a distri- bution which was adequate for an analysis ofa self-rating scale such as the MDI, i.e. a scalefor patients with mild to marked degrees of de-pressive states. All patients in the present study the coefficient for testing to what extent all the were able to complete the MDI, indicating a high dimensionality of the scale was acceptable (0.52).
The 10 items of the MDI obviously have a high Table 1 shows the rank-order of the MDI items content validity when compared to the diag- when using the mean score value for the indi- nostic systems (DSM-IV or ICD-10) as the scale vidual items as index of inclusiveness. Thus, at is based on the universe of symptoms within these the top is placed item 3 (lack of energy) and at the systems. Although symptoms with a high diag- nostic validity do not necessarily have a high The Rasch analysis confirmed that the 10 items validity for measuring severity (Frances et al.
of MDI constitute one dimension. According to 1990 ; Kessler & Mroczek, 1995), the present the Rasch analysis the same rank-order of the study showed that the MDI is a unidimensional individual items was found both when males scale. This was supported both with classical were compared with females and when patients psychometric tests (e.g. principal component with low total MDI scores were compared with analysis and Cronbach’s coefficient alpha) and patients with high total MDI scores. Where with modern psychometric tests (e.g. the Mok- discrepancies emerged in rank-order between ken analysis and the Rasch analysis). The rank- the Mokken analysis and the Rasch analysis the order of inclusiveness showed almost the same difference was only of the order of one rank. The pattern when applying the two different types of item with the lowest coefficient in the Mokken modern psychometric tests. The structure of in- analysis was item 9 (sleep), which also was the clusiveness shows that the core symptoms of depression according to DSM-IV and ICD-10(depressed mood, lack of energy and lack of in-terests) are among the most inclusive items of the MDI (Table 1) indicating a ‘ ceiling effect ’, while When the MDI scores were correlated to the the items of guilt feelings and suicidal thoughts were most exclusive indicating a ‘ floor effect ’.
ficient was 0.86 (Pf0.01), (the corresponding The somatic items (sleep and appetite) showed non-parametric Spearman coefficient was 0.80 suboptimal fitting in the Mokken analysis (Loevinger’s coefficients <0.40) as well as in the The MDI scale for measuring severity of depression Standardization of the Major Depression Inventory (MDI ) using the HAM-D17 as index of Probable major depression/mild depression Rasch analysis. Furthermore, in the principal depression to fulfil the objective of the study. The scale might perform differently in a more homo- showed the lowest factor loadings. However, the genous sample of depressed people, studies to in- somatic items had no impact on the overall val- spect this are now in progress. Additional items idity of the MDI indicating that the total score is could have been added, e.g. an item about hy- a sufficient statistic. The three self-rating scales persomnia, which is included in the DSM-IV but developed before the release of the DSM-III not in the ICD-10. Nevertheless, the purpose (BDI, SDS and CES-D) have all previously been with this scale was to make it as short as possible correlated with the HAM-D17 and coefficients while still covering enough information to make between 0.6 and 0.8 have been reported (e.g.
diagnoses as well as to rate severity of depression.
Brown & Zung, 1972 ; Bech et al. 1975 ; Biggs In conclusion, this study has shown that the et al. 1978 ; Radloff, 1977). The correlation co- total score of the MDI is a sufficient statistic to efficient of 0.86 found in the present study is, measure severity of depressive states. Moreover, a linear correlation to the Hamilton Depression The standardization of the MDI indicated that Scale has been found, resulting in a standardiz- a cut-off score of 27 corresponds to a score of 18 ation of the MDI by using the HAM-D17 as index on HAM-D17 (or major depression), which is in agreement with our analysis of the MDI whencompared to the diagnosis of major depressionbased on a psychiatric interview (Bech et al.
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Have you lost interest in your daily activities? Have you felt lacking in energy and strength ? Have you had a bad conscience or feelings of guilt ? Have you felt that life wasn’t worth living? Have you had difficulty in concentrating, e.g. when reading the newspaper orwatching television ? 10a Have you suffered from reduced appetite ? 10b Have you suffered from increased appetite ?

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