Manual for the Benzodiazepine Dependence Questionnaire (BDEPQ)

32 PROPERTIES OF BDEPQ TOTAL SCORES
Normality
A Lilliefors test (Lilliefors, 1967) was performed to examine the normality
of BDEPQ total scores. Scores from both the rst and second waves were not
normally distributed (Wave 1: Maximum di erence = 0:136�p < :05, Wave 2:
Maximum di erence = 0:111�p < :05). The skewness of BDEPQ scores was 0:906
at wave oneand0:889 at wave two. Wilkinson (1990) recommends 2 q 6
n as a
critical value for skewness, which is0:356 for wave one and 0:376 for wave two.
Comparison with these values indicates signi cant positive skewness in BDEPQ
scores. The kurtosis of total scores was 0:376 at wave one and 0:264 at wave
two. The signi cance of these values can be gauged by comparison using the same
formula (Wilkinson, 1990). This indicates that BDEPQ scores from wave one are
leptokurtic while those from wave two havea`peakedness' similar to the normal
distribution. The distribution of total BDEPQ scores from wave one is shown
graphically in Figure 1.
Internal consistency
Coe cient alphas for the total and sub-scale scores collected in the second
wave con rm the reliability of the scales. Because the information was collected
from the same respondents it is likely that regression toward the mean reduced the
variability of scores and in uenced the size of these coe cients from the second
wave.
The standard error of measurement of total and sub-scale BDEPQ scores is
shown in Table 8. These values were calculated with the formula described by
Ghiselli, Campbell, and Zedeck (1981, p. 206) and indicate the con dence that
can be placed in an individuals score on the scale. The table shows 4:47 as the error
of measurement for the rst wave. A 95% con dence interval around any score
on the BDEPQ would thus be 1:96 times this error of measurement (8:76). For
example if a person scored 60 on the BDEPQ prior to detoxi cation, completed
the scale six months later and scored 50, one could be 95% sure that this change
of 10 scale points was not due to chance or error in the scale. The estimates of
standard error of measurement from wave one should be used for any practical
purposes as regression to the mean may have reduced the variance in wave two
scores and consequently reduced the error of measurement.
Temporal stability of BDEPQ items
The three to four month test-retest correlation between scores on the 30-item
BDEPQ in wave one and wave two was 0:88 (n = 128). The 3 subscales show
similar test-retest coe cients of 0:83 for the general dependence subscale (n =
175), 0:73 for the pleasant e ects subscale (n = 160), and 0:87 for the perceived
need subscale. This indicates that the scale is measuring a relatively stable aspect
of the respondents attitudes. As discussed in the introduction dependence is best