In clinical trials, a<unk>  (or<unk>  marker) is a measure of effect of a specific treatment that may correlate with a real clinical<unk>  but does not necessarily have a guaranteed relationship. The National Institutes of Health (USA) defines<unk>  as "a biomarker intended to substitute for a clinical<unk> ".<unk>  markers are used when the primary<unk>  is undesired (e.g., death), or when the number of events is very small, thus making it impractical to conduct a clinical trial to gather a statistically significant number of<unk> s. The FDA and other regulatory agencies will often accept evidence from clinical trials that show a direct clinical benefit to<unk>  markers.<unk> s can be obtained from different modalities, such as, behavioural or cognitive scores, or biomarkers from Electroencephalography (qEEG), MRI, PET, or biochemical biomarkers. A correlate does not make a<unk> . It is a common misconception that if an outcome is a correlate (that is, correlated with the true clinical outcome) it can be used as a valid<unk>  (that is, a replacement for the true clinical outcome). However, proper justification for such replacement requires that the effect of the intervention on the<unk>  predicts the effect on the clinical outcome: a much stronger condition than correlation. In this context, the term Prentice criteria is used. The term<unk> " should not be used in describing<unk> s. Instead, descriptions of results and interpretations should be formulated in terms that designate the specific nature and category of variable assessed. A<unk>  of a clinical trial is a laboratory measurement or a physical sign used as a substitute for a clinically meaningful<unk>  that measures directly how a patient feels, functions or survives. Changes induced by a therapy on a<unk>  are expected to reflect changes in a clinically meaningful<unk> .
Surrogate endpoint
Clinical endpoint
Parallel study
Intersalt study
Multiple baseline design
Blinded experiment
N of 1 trial
Sensitivity training