Systematic or interval sampling


Presence of a gap, or interval between each selected unit in the sample. The following steps must be followed to obtain a systematic sample: (i) number the units of the sampling frame from 1 to N (where N is the total population size); (ii) determine the sampling interval (K) by dividing the number of units in the population by the desired sample size; (iii) select a number between one and K at random -this number is called the random start and would be the first number included in your sample; (iv) select every Kth (in this case, every fourth) unit after that first number.