There is always a compromise between including sufficient practitioners to recruit a representative cohort of patients and the time and cost involved in recruiting and maintaining the motivation of these practitioners.

Problems are more acute within primary care where, even for common conditions,the number of patients that practitioners see with the disease of interest represents only a small proportion of their total consultations.

Not all practices within a defined catchment area may agree to participate in a trial and the selected group of practitioners who agree to recruit patients can affect the representativeness of study population.

Practitioners with a particular interest in condition under investigation may be more likely to participate and may manage patients more effectively than the average practitioner, thus decreasing the effect size observed.