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Trials in primary care should recruit participants that are more representative of patients seen in the community than are in trials in secondary care. However, if the processes that operate to determine whether a patient is included in a trial are not random, trial participants may be skewed with respect to disease severity or other factors such as age or social class. Although this will not bias the trial result (internal validity), it may misrepresent the effect of the intervention in non-trial settings (external validity). Modelling, sensitivity analysis, and statistical estimates of uncertainty are necessary to determine the generalisability of the trial and to particularise results to a given clinical setting.(Lilford 1998)

Primary care provides many opportunities but is not an easy place to conduct research. Trials must be designed and undertaken by multidisciplinary teams with expertise in both the context of clinical practice and research methods.