Dj. Lawrence et al., Measuring the effectiveness of cataract surgery: the reliability and validity of a visual function outcomes instrument, BR J OPHTH, 83(1), 1999, pp. 66-70
Aims-To assess test-retest reliability and validity of the "TyPE" patient s
elf assessed visual function questionnaire, as part of a study in two hospi
tals measuring the effectiveness of cataract surgery. The American TyPE que
stionnaire had minor adaptations made for use in Britain.
Methods-Test-retest reliability was assessed on 63 out of 378 adult catarac
t surgery patients in the study, using Spearman correlation coefficients an
d kappa coefficients of agreement. "Construct" validity was evaluated by co
mparing the association between changes in visual function questionnaire sc
ores after surgery, with patients' perception of change in visual function
obtained by independent interview of 24 patients.
Results-The TyPE questionnaire items showed very good test-retest reliabili
ty. Average Spearman and kappa coefficients for 39 patients from hospital 1
were 0.93 and 0.84 respectively. Spearman and kappa coefficients of 0.9 an
d 0.81 were obtained for those nine patients in hospital 2 where both the t
est and retest questionnaires were filled in by the same people. However, f
or the 15 patients from hospital 2, where the questionnaire was filled in b
y different people in the retest, reliability was less good: the Spearman c
oefficients were still high, average 0.72, but the kappa coefficients were
poor, 0.27. Good construct validity was exhibited, with a correlation of 0.
79 between change in distance vision score from the questionnaires and the
independent interview.
Conclusions-The adapted TyPE questionnaire is both very reliable and has go
od construct validity. The kappa coefficient should be used wherever possib
le to evaluate reliability. The test-retest reliability and validity and pr
acticability of other visual function questionnaires have not been assessed
adequately, and further development should be carried out of all such ques
tionnaires, so that they may be introduced into routine clinical care.