estimation becomes much more complex. Since estimation become somewhat involved, attempts have been made to make
the estimation procedures somewhat simpler. We shall not get into vast area of sampling
literature in varying probability without replacement. However, it is worthwhile to mention
about a special category of varying probability without replacement procedure, which is
commonly used in Agricultural censuses as well as annual surveys. For sampling procedures
with varying probabilities without replacement, if inclusion probabilities of sampling units
are proportional to size measures, then estimation becomes very simple. Such procedures
are called Inclusion Probability Proportional to Size (IPPS) procedures or πps procedure,
since inclusion probabilities are sometimes denoted by s i π ' . One such procedure is PPS
systematic sampling which is described below.