The epidemiological diagnostic tests are often too expensive, invasive, or time-consuming for general population where the great majority will be negative. In order to validate the screening test results a sub-sample of the screened subjects can be drawn for testing by the accurate diagnostic test, a two-phase sampling design has been considered. It is sometimes convenient and economical to collect certain items of information on the whole of the units of a sample, and other items of information on only some of these units, these latter units being so chosen as to constitute sub-sample of the units of the original sample, this called two-phase sampling design.