Farm questionnaires have been established as a basic tool for General Surveillance of GMP. In surveys farmers are asked on their observations of specific monitoring characters to assess whether the areas of protection are influenced by the cultivation of GMP. Farmers give their assessment in comparison to conventional cultivation mainly in form of categorical answers (i.e. “normal”, “better”, “worse”). This method is easy to use, practical and effective. Nevertheless, its application needs a high standard of quality control. The methodology of empirical data survey knows several problems that may arise if its procedures are not applied properly. Wrong conclusions may result for example from interviewer effects (e.g. if interviewers are not sufficiently supervised), survey manager effects (e.g. if the design of the questionnaire is biased by the meaning or expectations of the project team), sample effects (e.g. if the design and sampling methods are not statistically sounded), but also from social effects (selective cognition, statements due to social norms) or from political effects (political pressure or expectations of the sponsor).
Empirical research has developed tools and quality standards to control such effects. Their use and a transparent exact documentation of the design, sampling, data and analytical methods of the questionnaires survey are an indispensable requirement for an empirical study to perform a General Surveillance of GMP.