Reliability refers to the consistency of results when the screening program is repeated on the same persons under the same conditions.  The clear definition of the endpoint and the standardization of screening methods should be respected to avoid any possible variation in results. The possible causes of variation in results when repeated screening is performed could reside in the biological variation of the actual manifestation being measured such as blood pressure which varies considerably for a given individual with time and under various circumstances.  Also, the precision of the instrument used could result in varying results as the standard mercury sphygmomanometer for blood pressure. Variability may also occur from differences in repeated measurements from the same screener or inconsistencies attributed to differences in the way different screeners apply or interpret the screening program results.