Briefly describe what is meant by "two-phase sampling," and discuss its advantages and disadvantages

What will be an ideal response?

In many investigations the population is not surveyed in a single step. Rather, it is often convenient to carry out an initial pilot study in which a relatively small proportion of the sample members are contacted. The results obtained are then analyzed prior to conducting the bulk of the survey. This approach is known as two-phase sampling.
The main disadvantage of such a procedure is that it can be quite time consuming. However, this factor may be outweighed by several advantages. One important benefit is that the investigator is able, at modest cost, to try out the proposed questionnaire in order to ensure that the various questions can be thoroughly understood. The pilot study may also suggest additional questions whose potential importance had previously been overlooked. Moreover, this study should also provide an estimate of the likely rate of nonresponse. Should this prove unacceptably high, some modification in the method of soliciting responses might appear desirable.
This approach has two further advantages. First, if stratified random sampling is employed, the pilot study can be used to provide estimates of the individual stratum variances. These, in turn, can be employed to estimate the optimum allocation of the sample among the various strata. Second, the results of the pilot study can be used to estimate the number of observations needed to obtain estimators of population parameters with a specified level of precision.