Effectiveness results from previous or concurrent studies of comparator therapies (including usual care or standard of care) are potentially useful for planning new studies, and for inclusion in indirect comparisons or meta-analyses that include results for the new medicine of interest. However, the populations included in these studies may have different characteristics to the target population for the new medicine. Differences may be related to demographics, risk profile, place in the treatment pathway, concomitant care received or referral practices. These may be a result of secular changes in patient management since these data were reported. As a result there may be variations in effectiveness reported for usual care in such trials or non-randomised studies, which may present problems for trial design (for example, using expected event rates to calculate adequate study size) as well as for meta-analyses synthesising results from relevant trials.