The validity of a screening program is measured by its sensitivity and specificity.  Sensitivity is the ability of screening to test positive for  truly affected people. Specificity is the ability of screening to test negative for truly unaffected people. The methods of calculating these measures are given in last slide. Obviously it is desirable to have a screening program that is both highly sensitive and highly specific. Usually this is not possible as there are some people who are clearly affected and some who are clearly unaffected but there are some who fall in between. In this situation, the cutoff between affected and unaffected is an arbitrary decision. Any screening program that has increased ability to avoid missing a true case will increase the number of unaffected individuals who will be wrongly picked as positive by the screening program.  Thus, whether we need a sensitive or a specific program depends on the condition and its burden on health. Generally we say that those skipped as negative by mistake (false negative) are important to avoid especially in severe conditions while those picked wrongly as positive (false positive) are just an alarm.