Positive control

A positive control is a comparison expected to produce a known response under the measurement or experimental system. It checks that the system can detect the effect it is meant to measure. For a generic assay, a reference sample known to contain the target should yield a signal; failure makes a negative test-sample result difficult to interpret. A positive control and a negative control answer complementary questions: whether the system can respond, and whether a response appears without the target cause.^[ref]