There is a turnover of volunteer observers in every scheme. Some volunteers stop after some time and others enter the scheme. Doesn´t this turnover affect the results, given that volunteers have different observational skills?
No, it does not.
First, differences between observation skills are minimized because all field workers in each monitoring scheme adopt a standardized field method.
Second, potential differences among fieldworkers are addressed in the TRIM models used to compute indices (in statistical terms: the site-effect is taken into account in the model and site-effects include differences between observers).
Third, most differences between volunteers will add to the noise in the data and not to bias in the results. Only if observational skills are changing in a systematic way, bias can be expected. So far, there are hardly any examples of systematic changes in the PECBMS monitoring results that are to be ascribed to systematic changes in the skills of volunteers. It is however an ongoing concern of monitoring coordinators to avoid any systematic changes in observer skills.