Validated questionnaires should not be modified.

T he reformatting and changes that were made to the Asthma Control Questionnaire (ACQ) [1] for the Post-cold Asthma Control and Exacerbation (PAX) study [2] raise some important concerns about modifying validated questionnaires. Just like mechanical and electrical measurement instruments, validated questionnaires are precision measurement instruments. The only difference being that they measure subjective rather than objective health status. Like any mechanical or electrical instrument, a great deal of care and expertise goes into the development of these questionnaires. Many studies have provided developers with the knowledge of how to: specify what the questionnaire is intended to measure (its construct); structure and select the right questions; formulate the responses; select the time specifications; optimise the page formulation for accurate completion; conduct validation studies (measurement properties and whether the instrument is measuring what it is meant to measure) and provide users the wherewithal to place a clinical interpretation on the data. For the same reason that one would never think of changing the numbers on the dial of a mechanical spirometer, one should never change a validated questionnaire. Even very small changes can destroy its validity. QUESTIONS Questions are selected by well-established methods (usually either ''importance'' or ''factor analysis'') [3] and their position in the questionnaire carefully ordered. Wording is checked for ease and accuracy of understanding (cognitive debriefing). For the analysis, each question has a weighting. For some questionnaires, this means that an algorithm must be used (e.g. The Short Form (SF)-36 Health Survey) [4], in others (e.g. the ACQ) questions are selected in such a way that they have equal weighting and the overall score is the mean of all the responses. The wording of questions should never be changed. Shortened versions should only be used when they have been validated [5]. Questions should never be added even if they are not going to be included in the analysis because they may alter how patients respond to other questions in the questionnaire. RESPONSES Most medical questionnaires use Likert-type scales (e.g. a seven-point scale ranging between 05no impairment and 65severe impairment). These are interval scales with equal spacing between each response. The verbal descriptor for each number is carefully chosen to enhance the perception of equal distance between the numbers. Patients use both the numbers and the words to select their responses (varying in preference between patients) and so equal prominence and position must be given to both. With careful …