Trials in primary care should recruit participants that are more representative of patients seen in the community than arein trials in secondary care. However, if the processesthat operate to determine whether a patient is included in a trialare not random, trial participants may be skewed with respectto 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-trialsettings (external validity). Modelling, sensitivity analysis,and statistical estimates of uncertainty are necessary to determinethe generalisability of the trial and to particularise resultsto 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 undertakenby multidisciplinary teams with expertise in both the contextof clinical practice and researchmethods.