Systematic sampling is a slight variation of simple random sampling in which only the first sample unit is selected at random and the remaining units are automatically selected in a definite sequence at equal spacing from one another. This technique of drawing samples is usually recommended if the complete and up-to-date list of the sampling units is available and the units are arranged in some systematic order such as alphabetical, chronological, geographical order, etc. This requires the sampling units in the population to be ordered in such a way that each item in the population is uniquely identified by its order, for example, the names of persons in a telephone directory, the list of voters, etc. We now list out some of the merits and demerits of the systematic sampling technique.
Advantages of Systematic Sampling:
- Systematic sampling is very easy to operate and checking can also be done quickly. Accordingly, it results in considerable saving in time and labour relative to simple random sampling or stratified random sampling.
- Systematic sampling may be more efficient than simple random sampling provided the frame is complete and up-to-date and the units are arranged serially in a random order like the names in a telephone directory where the units are arranged in alphabetical order. However, even in alphabetical arrangement, certain amount of non-random character may persist.
Disadvantages of Systematic Sampling:
- Systematic sampling works well only if the complete and up-to-date frame is available and if the units are randomly arranged. However, these requirements are not generally fulfilled.
- Systematic sampling gives biased results if there are periodic features in the frame and the sampling interval is equal to or a multiple of the period.
- The relative efficiency of the systematic sampling over stratified random sampling or simple random sampling without replacement (srswor) largely depends on the properties of the population under study. Without a knowledge of the structure of the population, no hard and fast rules can be laid down and no situations can be pinpointed where the use of systematic sampling is to be recommended.