MULTI-STAGE SAMPLING
Multi-stage sampling, as the name implies, involves the selection of units in more than one stage. In such a sampling, the population consists of a number of first stage units, called primary sampling units (PSUs). Each of these PSUs consists of a number of second-stage units. First, a sample is taken of the PSUs, and then a sample is taken of the second-stage units. This process continues until the selection of the final sampling units. It may be noted that at each stage of sampling, a sample can be selected with or without stratification.
An illustration would make the concept of multi-stage sampling clear. Suppose a sample of 5000 urban households from all over the country is to be selected. In such a case, the first stage sample may involve the selection of districts. Suppose 25 districts out of say 500 districts are selected. The second stage may involve the selection of cities, say four from each district. Finally, 50 households from each selected city may be chosen. Thus, one would have a sample of 5000 urban households, arrived at in three stages. It is obvious that the final sampling unit is the household.
In the absence of multi-stage sampling of this type, the process of the selection of 5000 urban households from all over the country would be extremely difficult. Besides, such a sample would be very thinly spread over the entire country and if personal interviews are to be conducted for collecting information, it would be an extremely costly affair. In view of these considerations a sampling from a widely spread population is generally based on multi-stage.
The number of stages in a multi-stage sampling varies depending on convenience and the availability of suitable sampling frames at different stages. Often, one or more stages can be further included in order to reduce cost. Thus, in our earlier example, the final stage of sampling comprised 50 households from each of the four selected cities. Since this would involve the selection of households all over the city, it would turn out to be quite expensive and time consuming if personal interviews are to be conducted. In such a case, it may be advisable to select two wards or localities in each of the four selected cities and then to select 25 households from each of the 2 selected wards or localities. Thus, the cost of interviewing as also the time in carrying out the survey could be reduced considerably.
It will be seen that an additional stage comprising wards or localities has been introduced here. Thus the sample has become a four-stage sample –
1st stage – districts
2nd stage – cities
3rd stage – localities
4th stage – households
From the preceding discussion it should be clear that a multi-stage sample results in the concentration of fieldwork. This in turn, leads to saving time, labor and money. There is another advantage in its use. Where a suitable sampling frame covering the entire population is not available, a multi-stage sample can be used.
32 Comments