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The second category of requirements deals with the screening test itself. Its suitability refers to its characteristics, such as its sensitivity and specificity. There are no absolute values for these characteristics, however, tests with poor characteristics should not generally be promoted. Acceptability of the test is very important. For example, colorectal screening involves the fecal occult blood test, which is not acceptable to many individuals. A population wide program may achieve very poor compliance. In the past it has been suggested that eventually it may be possible to screen people for a whole series of diseases at one time only. In fact, screening programs usually must be conducted on a continuing basis. An important part of screening policy is deciding how often a test should be done.