Steps validating questionnaire
Where a questionnaire results in a simple score, treatment effects can be assessed by examining pre- and post-treatment differences between the intervention and control group by means of unpaired t-tests or repeated measures analysis of variance.As additional evidence, patients’ perceptions of change can also be measured and relationships between reported change and difference in quality of life scores can be examined. Outside randomised controlled trials, where there may be baseline differences between treatment groups, analyses of covariance may be more appropriate for assessing responsiveness.Clearly, a questionnaire that cannot demonstrate that responses are stable over a short period of time in a pre-treatment sample will not be able to measure change following treatment accurately.
While it is acknowledged that urodynamic studies or pad tests represent the most accurate representation of leakage and thus of a clinical diagnosis of incontinence, these factors are not the only ones that one would want to be reflected by a questionnaire.
A Cronbach’s alpha in excess of 0.70 is usually considered to show adequate internal consistency.
Particularly important for questionnaires used to examine outcome is the concept of stability - whether the questionnaire measures the same sorts of things in the same person over a period of time.
Internal consistency can be assessed by statistical techniques such as item-total correlations or Cronbach’s alpha coefficient.
Cronbach’s alpha coefficient should be calculated for the total score eliminating one item at a time, and any items that significantly increase or decrease the alpha should be re-evaluated.