Systematics Sampling -Criteria to choose -selecting a systematic sample

In systematic sampling the sample units are selected from the population at equal intervals in terms of time, space or order. The selection of a sample using systematic sampling method is very simple. 

From a population of ‘N’ units, a sample of ‘n’ units may be selected by following the steps given below:

1. Arrange all the units in the population in an order by giving serial numbers from 1 to N.
2. Determine the sampling interval by dividing the population by the sample size. That is, K=N/n.
3. Select the first sample unit at random from the first sampling interval (1 to K).
4. Select the subsequent sample units at equal regular intervals.

 For example:

1. we want to have a sample of 100 units from a population of 1000 units. First arrange the population units in some serial order by giving numbers from 1 to 1000. 
2. The sample interval size is K=1000/100=10. Select the first sample unit at random from the first 10 units ( i.e. from 1 to 10). 
3. Suppose the first sample unit selected is 5, then the subsequent sample units are 15, 25, 35,.........995. 
4. Thus, in the systematic sampling the first sample unit is selected at random and this sample unit in turn determines the subsequent sample units that are to be selected

Advantages

i) The main advantage of using systematic sample is that it is more expeditious to collect a sample systematically since the time taken and work involved is less than in simple random sampling. For example, it is frequently used in exit polls and store consumers.

ii) This method can be used even when no formal list of the population units is available. For example, suppose if we are interested in knowing the opinion of consumers on improving the services offered by a store we may simply choose every kth (say 6th) consumer visiting a store provided that we know how many consumers are visiting the store daily (say 1000 consumers visit and we want to have 100 consumers as sample size).

Limitations

i) If there is periodicity in the occurrence of elements of a population, the selection of sample using systematic sample could give a highly unrepresentative sample. For example, suppose the sales of a consumer store are arranged chronologically and using systematic sampling we select sample for 1st of every month. The 1st day of a month can not be a representative sample for the whole month. Thus in systematic sampling there is a danger of order bias.

ii) Every unit of the population does not have an equal chance of being selected and the selection of units for the sample depends on the initial unit selection. Regardless how we select the first unit of sample, subsequent units are automatically determined lacking complete randomness.