Very few head-to-head, peer-reviewed trials exist; instead, evidence for efficacy of treatments is often ascertained from placebo-controlled trials, registries and meta-analyses, which often do not sufficiently address the relative effectiveness of two medications.
Area-based measures, such as the Townsend Index of Deprivation[ 6], are often more easily ascertained than individual-level measures of deprivation, and are likely to reflect important area-level determinants of health and lifestyle.
Despite the popularity if a given traditional Chinese medicine, the actual contents — frequently in the form of a pill, powder or tea — are often hard to ascertain.
Du Châtelet thinks hypotheses are useful (in addition to their providing a necessary step in scientific method) because "when a hypothesis is once posed, experiments are often done to ascertain if it is a good one, experiments which would never have been thought of without it" (IP §58).
However, the virus is not bound by vows of fidelity and behavioral characteristics of sexual partnerships are often difficult to ascertain due to recall bias and willingness of some persons to report sensitive behaviors such as concurrent partnerships.
Biologic markers are often used to ascertain validity of reported data (38 ).
Furthermore, within the hospital setting, rule-based criteria are often applied to ascertain if an outbreak has occurred [ 2].
However, the true incidence rates are often difficult to ascertain, due in part to difficulties in obtaining death certificates as well as the accuracy on the certificate itself (SCD is often unwitnessed).
When considering transmission events in discordant couples, isolating the effect of STI co-infections is challenging because concomitance of STI infections in both partners (that could affect both HIV susceptibility and infectiousness) and HIV transmission are often not practical to ascertain.
Rates by surgeon or by hospital can be difficult to ascertain because patients are often operated by different surgeons or in different hospitals [ 13].
