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Questionnaires in epidemiological studies must have high validity and reliability. A preliminary pilot study with a few participants (roughly 30-50) should always be conducted in order to estimate the validity and the reliability of a questionnaire and to correct as far as possible any errors and omissions detected in this way. The validity of a questionnaire is the extent to which the questionnaire measures the study concept or, otherwise, the variate that it is supposed to measure. Increase in validity of a questionnaire is associated with a decrease in systematic error. Evaluation of the validity of a questionnaire includes evaluation of content validity, construct validity, criterion validity and face validity. Content validity is the extent to which a questionnaire covers the different dimensions of the concept that is being measured. Construct validity is the extent to which a questionnaire reflects the real theoretical meaning of the concept that it is supposed to measure. Estimation of construct validity can be performed by factor analysis, convergent validity, discriminant validity and the known groups' method. Factor analysis is a complex mathematical procedure, comprising explanatory factor analysis and confirmatory factor analysis. Criterion validity is about the use of a criterion, i.e., a questionnaire with proved validity and reliability, as a reference method or "gold standard" for estimation of the validity of a new questionnaire; the new and the proven questionnaires must measure the same concept. The reliability or precision of a questionnaire concerns the degree of stability or consistency with which the questionnaire measures the concept that it is supposed to measure. Increase in the reliability of a questionnaire is associated with a decrease in random error. The estimation of reliability of a questionnaire includes test-retest estimation, alternative forms reliability, split-half reliability, internal consistency reliability and inter-observer reliability. [ABSTRACT FROM AUTHOR]

In epidemiological studies the use of appropriate questionnaires for essential data collection is fundamental for achieving validity and reliability. The researchers must firstly determine with clarity their research hypothesis, in order to create and use the appropriate questionnaire in an effective way. The questionnaire items must be relevant to the research hypothesis of the study. In order to achieve this it is necessary to conduct a systematic literature review, on the basis of which the researchers can design their study in an appropriate way with use of the appropriate questionnaire. After creation of a new questionnaire or the choice an instrument already in use, a pilot study must be conducted with a relatively small number of participants (roughly 30-50), in order to make a preliminary estimation of the validity and reliability of the questionnaire and to correct as far as possible its errors and failures. The first part of each questionnaire should be a cover letter, which is very important since it is the first contact of the candidate participants with the study and the questionnaire that they are required to answer. The cover letter should be brief, explicit and comprehensive, encouraging the reader to complete the questionnaire and participate in the study. Questionnaires usually consist of "closed" and or "open" questions. "Open" questions have no predefined answers, but allow the participants every possible response. Concerning "closed" questions, the participants have to choose between answers predefined by the researchers. "Closed" questions are used mainly in quantitative research, while "open" questions are used mainly in qualitative research. The format of a questionnaire is very important in order for it to be answered appropriately by candidate participants. A study questionnaire in a study should ideally be short, comprehensive and explicit, in order to increase the response rate of the participants. [ABSTRACT FROM AUTHOR]