This method is also known as multi stage sampling .Under this method random selection is made of the ultimate or final units from a given stratum. The sampling process is carried out in several stages. At first stage samples are selected by some suitable method. Then the second sample out of the first be chosen again by some suitable method. Further stages may be added as per the requirements.
This method introduces flexibility in the sampling method which is lacking in other methods; it enables existing divisions of universe to be used as units of various stages. The large area can be convened under this method.
Under this method a sample is less accurate as compared to a sample of the same a number of final stage units which are selected by some suitable single stage process.