Kish mentions that a good sample design requires the judicious balancing of four broad criteria-goal orientation, measurability, practicality and economy.
Goal Orientation: This suggests that a sample design “should be oriented to the research objectives, tailored to the survey design, and fitted to the survey conditions”. If this is done, it should influence the choice of the population, the measurement as also the procedure of choosing a sample.
Measurability: A sample design should enable the computation of valid estimates of its sampling variability. Normally, this variability is expressed in the form of standard errors in surveys. However, this is possible only in the case of probability sampling. In non-probability samples, such as a quota sample, it is not possible to know the degree of precision of the survey results.
Practicality: This implies that the sample design can be followed properly in the survey, as envisaged earlier. It is necessary that complete, correct, practical and clear instructions should be given to the interviewer so that no mistakes are made in the selection of sampling units and the final selection in the field is not different from the original sample design. Practicality also refers to simplicity of the design, i.e. it should be capable of being understood and followed in actual operation of the field work.
Economy: Finally, economy implies that the objectives of the survey should be achieved with minimum cost and effort. Survey objectives are generally spelt out in terms of precision, i.e. the inverse of the variance of survey estimates. For a given degree of precision, the sample design should give the minimum cost. Alternatively, for a given per unit cost, the sample design should achieve maximum precision (minimum variance).
It may be pointed out that these four criteria come into conflict with each other in most of the cases, and the researcher should carefully balance the conflicting criteria so that he is able to select a really good sample design. As there is no unique method or procedure by which one can select a good sample, one has to compare several sample designs that can be used in a survey.
This means that one has to weigh the pros and cons, the strong and weak points of various sample designs in respect of these four criteria, before selecting the best possible one.
Sample Size Decisions: After having looked into major sample designs, we now turn to another important aspect of sampling, namely, the sample size. When a survey is undertaken and when it is not possible to cover to entire population, the marketing researcher has to answer a basic question – How large should the sample be? We will focus our attention on this basic problem and discuss how decisions on sample size are taken.
