Q1. What are the criteria needed to legitimize what the questionnaire is?
Explicit quality criteria for studies on the development and evaluation of health status questionnaires are needed to legitimize what the best questionnaire is.
Q2. How many patients are adequate for the assessment of the agreement parameter?
In both cases, the authors consider a sample size of at least 50 patients adequate for the assessment of the agreement parameter, based on a general guideline by Altman .
Q3. Why do the authors prefer the SEM to be considered part of the measurement error?
The authors believe that systematic differences should be considered part of the measurement error, because the authors want to distinguish them from ‘‘real’’ changes, e.g., due to treatment.
Q4. Why is the number of health status questionnaires growing?
Because the number of health status questionnaires is rapidly growing, choosing the right questionnaire for a specific purpose becomes a time-consuming and difficult task.
Q5. How many patients are in correspondence with the hypotheses?
The authors therefore give a positive rating for construct validity if hypotheses are specified in advance and at least 75% of the results are in correspondence with these hypotheses, in (sub)groups of at least 50 patients.
Q6. What can be done to make an evidence-based choice for the questionnaire?
With this table one can make an evidencebased choice for the questionnaire with the best measurement properties, taking into account those measurement properties that are most important for a specific application (e.g., reliability when using a questionnaire for discrimination and responsiveness when using it for evaluation of a treatment effect) and the population and setting in which the questionnaire is going to be used.
Q7. What is the way to measure internal consistency?
The authors give a positive rating for internal consistency when factor analysis was applied and Cronbach’s alpha is between 0.70 and 0.95.
Q8. What is the way to measure agreement?
The authors give a positive rating for agreement if the SDC (SDCind for application in individuals and SDCgroup for use in groups) or the limits of agreement (upper or lower limit, depending on whether the interest is in improvement or deterioration) are smaller than the MIC.