A<unk>  marker (or<unk>  is a term used in medical research for a change to the human body that has been documented to be involved in the causal pathway and necessary for an eventual outcome or<unk>  to occur.
In clinical trials, a<unk>  is a measure of the effect (often what is known as a biomarker like cholesterol) of a certain treatment that is substituted in the evaluation of a new drug or device in place of a "hard<unk> " like death or heart attack. The National Institutes of Health (USA) defines<unk> s as: "A biomarker intended to substitute for a clinical<unk> ". The use of a<unk>  can lead to more rapid and efficient completion of clinical trials, but the use of a<unk>  has been criticized as reductions of a substitute maker are not always predictive of improvements in clinical outcomes. The classic example of a failed<unk>  marker is the use of PVC suppression as a substitute marker of anti-arrhythmia effectiveness. Class III anti-arrhythmias reduced PVCs or extra heart beats, but were associated with a higher risk of death in the Cardiac Arrhythmia Suppression Trial (CAST).<unk>   markers may be used when it is unethical to look for the<unk>  (e.g.,  death) in the experiment, or when the number of<unk>  events is very  small, thus making it impractical to conduct an experiment to look for  the<unk> .  The measurement of<unk>  markers provides a way to  test the effectiveness of a treatment for a fatal disease without having  to wait for a statistically significant number of deaths to occur. the FDA will often accept evidence from clinical trials that show a benefit to<unk>  markers instead of to<unk> s.  As stated by Thomas Fleming: "The accelerated-approval regulatory process is intended to address this need by allowing marketing of interventions shown to have strong effects on measures of biological activity, if those measures are potential<unk> s" for true measures of tangible clinical benefit."
Surrogate endpoint
Block design
Stratified sampling
Propensity score
Evidence-based design
Selectable marker
Agent study
Biased sample