The information obtained in a study should be valid, i.e. reflecting the true situation (1).
A questionnaires is considered validated if the questionnaire has been shown to have a high reliability and internal consistency. A questionnaire with a high reliability would elicitate the same answers if applied to the same population again. Internal consistency is measured by comparing the answers to questions measuring the same concepts. Validated questionnaires exist for a large number of study questions, for example quality of life, pain and chronic diseases. Validated questionnaires are widely used in social sciences.
The ideal would be to use validated questionnaires in all investigations. In intervention epidemiology, however, validated questionnaires are very uncommon. In field epidemiology, epidemiologists rely frequently on already used standard questionnaires, for example for food-borne outbreaks. However, the situation differs for each outbreak, each study and each country. Re-using standard questionnaires will not necessarily point towards the exposure of interest. Therefore you need to generate a hypothesis first using a trawling questionnaire. This will help you to design an appropriate questionnaire for your study.
1. Silman AJ, MacFarlane GJ :Epidemiological Studies. A practical guide. 2nd edition 2002, Cambridge. Pp 111-122