In systematic sampling, the first sample unit is rendomly selected and the remaining units of the sample are automatically selected in definite sequence at uniform interval from the list of population units.

• If the complete list of population units arranged in some systematic manner is available, then this method of sampling is advisable.
• Suppose, N units of the population are arranged in some systematic manner and numbered 1 to N. A sample of size n is to be drawn. Then sampling interval k = Nn; where k is a positive integer is determined and a random number is selected from first k units of population and select every kth unit therefore.
• Set of such selected units is called systematic sample and the method of obtaining such a sample is called systematic sampling.

Illustration: If the random number selected from the first k unit is 4, then the units at order 4, 4 + k, 4 + 2k, 4 + 3k, …, etc. of the list of population units form the systematic sample.