In many surveys, the population can be divided into groups of units. One approach is to treat these groups as strata and select a separate sample from each group. An alternative approach is to treat these groups as clusters, in which case only a sample of them is included in the survey.
Multi-Stage Sampling: Definition, Process, Stages Explained
Multi-Stage Sampling Definition
The multi-stage sampling method is regarded as cluster sampling if all the units in the selected clusters are included in the sample. These two approaches have already been discussed. If only a sample of units is selected from each selected cluster, the method is known as two-stage sampling.
What is The Primary Sampling Unit (PSU)?
Frequently, a hierarchy of clusters is used. Initially, a population is divided into several large clusters of elements, called primary sampling units (PSU).
What is The Sampling Unit (SSU)?
Following this, several primary sampling units are selected at random, and from those selected primary sampling units, some smaller units, called secondary sampling units (SSU), are selected again at random.
The process continues until units are selected within the ultimate units. As the selection of the sample is made in more than one stage, it is known as multi-stage sampling.
Multi-Stage Sampling Process
In multi-stage sampling, the primary sampling unit (PSU) is generally a compact geographical area, for instance.
The second stage unit (SSU) may, in that case, be a local unit, such as a village in a rural areas block of contiguous housing units) in urban areas.
The next selection stage may involve the selection of households or housing units. If no further stage is needed, then the elements in this stage are called the ultimate sampling unit (USU), and the second stage units may be described as the penultimate sampling unit (PUSU).
The multi-stage sampling is, in essence, a process of sub-sampling.
Stages of Multi-Stage Sampling
- Once the primary sampling units are selected at the first stage, the subsequent stages are nothing but a sub-sampling process from the units chosen at the immediate previous stages.
- Thus the second stage sample of units is chosen from the first stage sample of units.
- The third stage sample of units is chosen from the second stage sample of units, and so on.
In multi-stage sampling, the fewer the stages of selection, the better and more efficient the sample would be.
A sampling at each stage, down to the penultimate stage, can be done by a variety of procedures, e.g., simple random sampling, sampling with probability proportional to size (PPS), systematic sampling, and systematic sampling with PPS.
At the ultimate stage, USUs within selected PUSU can also be selected by different methods; one can select a simple random sample or a stratified random sample of USUs, or one can divide the PUSU into clusters of households and select a number of clusters.
Generally, PSUs, SSUs, and other intermediate stage units down to PUSUs are selected with probability proportional to size. USUs are selected by simple random sampling or systematic sampling with equal probability.
In some instances, the USUs are sampled as clusters of contiguous units.