Friday, 12 July 2013



It is the procedure required right from defining a population to the actual selection of sample elements.There are seven steps involved in this process.

Step 1: Define the population
It is the aggregate of all the elements defined prior to selection of the sample. It is necessary to define population in terms of
(i)        elements
(ii)      sampling units
(iii)    extent
(iv)    time.

It may be emphasized that all these four specifications must be contained in the designated population Omission of any of them would render the definition of population incomplete

Step 2 : Identify the sampling frame
Identifying the sampling frame, which could be a telephone directory, a list of blocks and localities of a city, a map or any other list consisting of all the sampling units. It may be pointed out that if the frame is incomplete or otherwise defective, sampling will not be able to overcome these shortcomings

The question is—How to ensure that the frame is perfect and free from any defect Leslie Kish has observed that a perfect frame is one where “every element appears on the list separately, once only once, and nothing else appears on the list” This type of perfect frame would indicate one-to-one correspondence between frame units and sampling units But such perfect frames are rather rare Accordingly, one has to use frames with one deficiency or another, but one should ensure that the frame is not too deficient so as to be given up altogether

This raises a pertinent question -What are the criteria for a suitable frame? In order to examine the suitability or otherwise of a sampling frame, a number of questions need be asked. These are
1 Does it adequately cover the population to be surveyed?
2 How complete is the frame? Is every unit that should be included represented?
3 Is it accurate? Is the information about each individual unit correct? Does the frame as a whole contain units, which no longer exist?
4 Is there any duplication? If so, then the probability of selection is disturbed as a unit can enter the sample more than once
5 Is the frame up-to-date? It could have met all the criteria when compiled but could well be deficient when it came to be used This could well be true of all frames involving the human population as change is taking place continuously
6 How convenient is it to use? Is it readily accessible? Is it arranged in a way suitable for sampling? Can it easily be re-arranged so as to enable us to introduce stratification and to undertake multi-stage sampling?
These are demanding criteria and it is most unlikely that any frame would meet them all Nevertheless, they are the factors to be borne in mind whenever we undertake random sampling
In marketing research most of the frames are from census reports, electoral registers, lists of member units of trade and industry associations, lists of members of professional bodies, lists of dwelling units maintained by local bodies, returns from an earlier survey and large scale maps.

Step 3: Specify the sampling unit
The sampling unit is the basic unit containing the elements of the target population. The sampling unit may be different from the element. For example, if one wanted a sample of housewives, it might be possible to have access to such a sample directly. However, it is easier to select households as the sampling unit and then interview housewives in each of the households.

As mentioned in the preceding step, the sampling frame should be complete and accurate otherwise the selection of the sampling unit might be defective. It is necessary to get a further specification of the sampling unit both in personal interviews and in telephone interviews. Thus, in personal interviews, a pertinent question is—of the several persons in a household, who should be interviewed? If interviews were held during office timings when the heads of families and other employed persons are away, interviewing would under-represent employed persons and over-represent elderly persons, housewives and the unemployed. In view of these considerations, it is necessary to have a random process of selection of the adult residents of each household. One method that could be used for this purpose is to list all the eligible persons living at a particular address and then select one of them.

Step 4: Specify the sampling method
It indicates how the sample units are selected. One of the most important decisions in this regard is to determine which of the two—probability and non-probability sample—is to be chosen.

In case of a probability sample, the probability or chance of every unit in the population being in the sample is known. Further, the selection of specific units in the sample depends entirely on chance. No substitution of one unit for another is permissible. This means that no human judgment is involved in the selection of a sample. In contrast, in a non-probability sample, the probability of inclusion of any unit in the population in the sample is not known. In addition, the selection of units within a sample involves human judgment rather than pure chance.

In case of a probability sample, it is possible to measure the sampling error and thereby determine the degree of precision in the estimates with the help of the theory of probability. This theory also enables us to consider, from amongst the various possible sample designs, the one that will give the maximum information per rupee. This is not possible when a non-probability sample is used.
Probability sampling enables us to choose representative sample designs. It also enables us to estimate the extent to which the results based on such a sample are likely to be different from what we would have obtained had we covered the population in our study. Conversely, the use of probability sampling enables us to determine the sample size for a given degree of precision, indicating that our sample results do not differ by more than a specified amount from those yielded by a study covering entire population.

Although non-probability sampling does not yield these benefits, on account of its convenience and economy, it is often preferred to probability sampling. If the researcher is convinced that the risks involved in the use of a non-probability sample are more than offset by its being relatively cheap and convenient, his choice should be in favor of non-probability sampling.

There are various types of sample designs that can be covered under the two broad groups, random or probability samples and non-random or non-probability samples.

Step 5: Determine the sample size
In other words, one has to decide how many elements of the target population are to be chosen.

Step 6: Specify the sampling plan
This means that one should indicate how decisions made so far are to be implemented. For example, if a survey of households is to be conducted, a sampling plan should define a household, contain instructions to the interviewer as to how he should take a systematic sample of households, advise him on what he should do when no one is available on his visit to the household, and so on. These are some pertinent issues in a sampling survey to which a sampling plan should provide answers
Step 7: Select the sample
This is the final step in the sampling process. A good deal of office and fieldwork is involved in the actual selection of the sampling elements. Most of the problems in this stage are faced by the interviewer while contacting the sample-respondents.


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