A number of epidemiologic designs are utilized to evaluate screening programs, including correlation studies, observational analytical studies and randomized trials. Correlation studies have been used to examine disease trends in relation to screening frequencies within a population or to compare the relationship between the frequencies of screening and disease rates for different populations. Correlation studies are useful in suggesting a relationship between screening and decline in morbidity or mortality. However, since the information from these studies concerns populations rather than individuals, it is not possible to identify that those experiencing the benefit are the same persons who received screening. Also, such studies cannot allow for control of potential confounding risk factors. Finally the measure of screening frequency is usually an average value for the population, so it is impossible to determine an optimal screening strategy for an individual.