THE SAMPLING PROCESS
Step 1: Define the Population
A population must be defined in terms of elements, sampling units, extent and time. IN relation to these constituent parts, the population of purchasing agent is.
purchasing agents in
(sampling unit) companies and governmental agencies that have
(extent) bought any of our products
(time) in the last three years
Step 2: Specify the Sampling Frame
If a probability sample is to be taken, a sampling frame is required. A sampling frame is a means of representing the elements of the population. A sampling frame maybe a telephone book, a city directory, an employee roster, a listing of all students attending a university, or a list of all possible phone numbers.
Maps also serve frequently as sampling frames. A sample of areas within a city may be taken and another sample of households may then be taken within each area. City blocks are sometimes sampled and all households on each sample block given instructions as to how to take “random walks” from the intersection and select the households to be interviewed.
A perfect sampling frame is one in which every element of the population is represented once but only once.
Step 3: Specify Sampling Unit
The sampling unit is the basic unit containing the elements of the population to be sampled. It may be the element itself or a unit in which the element is contained. For example, if one wanted a sample of males over 13 years of age, it might be possible to sample them directly. In this case, the sampling unit would be identical with the element. However, it might be easier to select households as the sampling unit and interview all males over 13 years of age in each household. Here the sampling unit and the population element are not the same.
The sampling unit selected is often dependent upon the sampling frame. If a relatively complete and accurate listing of elements is available – register of purchasing agents, for example – one may well want to sample them directly. If no such register is available, one may need to sample companies as the basic sampling unit.
Step 4: Selection of Sampling Method
The sampling method is the way the sample units are to be selected. Five basic choices must be made in deciding on a sampling method:
Probability versus nonprobability,
Single unit versus cluster of units,
Unstratified versus stratified,
Equal unit probability versus unequal unit probability, and
Single stage versus multistage.
Step 5: Determination of the Sample Size
The determination of the proper sample size has traditionally been taught by one method in statistics classes and often practiced by an entirely different approach in the field. The reason for this is that traditional sampling theory generally ignores the concept of the cost versus the value of the information to be provided by various sized samples. Practitioners have been forced to deal with the realities of sampling economics regardless of whether theory recognizes them.
Step 6: Specify the Sampling Plan
The sampling plan involves the specification of how each of the decisions made thus far is to be implemented. It may have been decided that the household will be the element and the block the sampling unit. How is a household defined operationally? How is the interviewer to be instructed to distinguish between families and households in instances where two families and some distant relatives of one of them are sharing the same apartment? How is the interviewer to be instructed to take a systematic sample of households on the block? What should the interviewer do when a housing unit selected is vacant? What is the callback procedure for households at which no one is at home? What age respondent speaking for the household is acceptable?
Step 7: Select the Sample
The final step in the sampling process is the actual selection of the sample elements. This requires a substantial amount of office and fieldwork, particularly if personal interviews are involved.