All eligible patients may not be prepared to enter, or may not be asked to enter, the study.

Observed variability in recruitment rates may be due to differences in the prevalence of disease or presentation rates but, variability in recruitment rates is likely to be also due to differences in proportion of eligible patients who were recruited. Interpretation of these differences requires access to records of all patients within participating practices who have the relevant disease but, it is rarely possible to obtain consent to access records from all patients not entering trials.

The study denominator and the representativeness of the sample can be determined by comparing patient and disease related characteristics of participants with those of total eligible population using anonymised data. However, this is not always feasible as not all practices routinely record all consultations on computer and manual searches of paper records are costly.