It is a probability sampling method in which sample members from a larger population are selected according to a random starting point and a fixed, periodic interval. This is possible only when the population size is finite, e.g., a systematic sampling of 100 data points in a population of 1,000 would involve observing every 10th data point. Area sampling is done when no complete frame of reference is available; purposive sampling is judgemental in logically assuming he sample to be representative of the population.