A clinical trial endpoint is defined as a measure that allows us to decide whether the null hypothesis of a clinical trial should be accepted or rejected (Bakhai et al., 2006a). In a clinical trial, the null hypothesis states that there is no statistically significant difference between two treatments or strategies being compared with respect to the endpoint measure chosen.
Clinical trial endpoints can be classified as primary or secondary.
Primary endpoints measure outcomes that will answer the primary (or most important) question being asked by a trial, such as whether a new treatment is better at preventing disease-related death than the standard therapy. In this case, the primary endpoint would be based on the occurrence of disease-related deaths during the duration of the trial. The size of a trial is determined by the power needed to detect a difference in this primary endpoint.
Secondary endpoints ask other relevant questions about the same study; for example, whether there is also a reduction in disease measures other than death, or whether the new treatment reduces the overall cost of treating patients. When secondary endpoints are also important the trial must be powered sufficiently to detect a difference in both endpoints, and expert statistical and design advice may be needed.
An endpoint could take different forms:
Ideally, a trial should have a single endpoint based on just one outcome measure. However, as the art of trial design has evolved, most large trials have a primary (composite) endpoint consisting of multiple outcome measures. An endpoint can also be the time taken for an event to occur. For such an endpoint, the events of interest for which a time is to be recorded—such as stroke or heart attack—must be predefined. Trial endpoints can also be a quantitative measurement of a biochemical or socioeconomic parameter such as cholesterol level or quality-of-life.