A sampling frame. is a representation of the elements of the target population, It consists of a list or set of directions for identifying the target population. Examples of a sampling frame include the telephone book, an association directory listing the firms in an industry, a mailing list purchased from a commercial organization, a city directory, or a map. If a list cannot be compiled, then at least some directions for identifying the target population should be as random digit dialing procedures in telephone surveys, In the opening duck stamp example, the sampling frame consisted of a computer program for randomly and efficiently generating telephone numbers, excluding nonworking and non household numbers.
Often it is possible to compile or obtain a list of population elements, but the list may omit some elements of the population or include other elements that do not belong. Therefore, the use of a list will lead of sampling frame error, the discrepancy between the population and the sampling frame is small enough to ignore. However, in.most-cases, the researcher should recognize and treat the sampling frame error. This can be done in at least three ways, One approach is to redefine the population in terms of the sampling frame. If the telephone book is used as a sampling frame, the population of households could be redefined as those with a correct listing in the telephone book in a given area. Although this approach is simplistic, it does prevent. the researcher from being misled about
the actual population being investigated.
Another way is to account for sampling frame error by screening the respondents in the data-collection phase, The respondents could be screened with respect to demographic characteristics, familiarity, product usage, and other characteristics to ensure that they satisfy the criteria for the target population. Screening can eliminate inappropriate elements contained in the sampling frame, but it cannot account for elements that have been omitted.
Yet another approach is to adjust the data collected by a weighting scheme to counterbalance the sampling frame error, Regardless of which approach is adopted, it is important to recognize any sampling frame error that exists, so that inappropriate population inferences can be avoided.
Select a Sampling Technique
Selecting a sampling technique involves several decisions of a broader nature. The researcher must decide whether to use a Bayesian or traditional sampling approach, to sample with or without replacement, and to use non probability or probability sampling.
In the Bayesian approach, the elements are selected sequentially. After each element is added to the sample, the data are collected, sample statistics computed, and sampling costs determined, The Bayesian approach explicitly incorporates prior information about population parameters as well as the costs and probabilities associated with making wrong decisions, This approach is theoretically appealing. Yet it is not used widely in marketing research because much of the required information on costs and probabilities is not available. In the traditional sampling approach, the entire sample is selected before data collection begins. Because the traditional approach is most commonly used, this is the approach assumed in the following sections.
In sampling with replacement, an element is selected from the sampling frame and appropriate data are obtained. Then the element is placed back in the sampling frame. As a result, it is possible for an element to be included in the sample more than once. In sampling without replacement, once an element is selected for inclusion in the sample, it is removed from the sampling frame and, therefore, cannot be selected again, The calculation of statistics is done somewhat differently for the two approaches, but statistical inference is not very different if the sampling frame is large relative to the ultimate sample size, Thus the distinction is important only when the sampling frame is not large compared to the sample size, The most important decision about the choice of sampling technique is whether to use probability or non probability sampling.
If the sampling unit is different from the element, it is necessary to specify precisely how the elements within the sampling unit should be selected, In in-home personal interviews and telephone interviews, merely specifying the address or the telephone number may not be sufficient, For example, should the person answering the doorbell or the telephone be interviewed, or some one else in the household? Often, more than one person in a household may qualify, For example, both the male and female heads of household may be eligible to participate in a study examining family leisure-time activities, When a probability sampling technique is being employed, a random selection must be made from all the eligible persons in each household, A simple procedure for random selection is the next birthday method, The interviewer asks which of the eligible persons in the household has the next birthday and includes that person in the sample, as in the opening duck stamps example.
Determine the Sample Size
Sample size refers to the number of elements to be included in the study, Determining the sample size is complex and involves several qualitative and quantitative considerations, Important qualitative factors that should be considered in determining the sample size include (1) the importance of the decision, (2) the nature of the research, (3) the number of variables, (4) the nature of the analysis, (5) sample sizes used in similar studies, (6) incidence rates, (7) completion rates, and (8) resource constraints.
In general, for more important decisions, more information is necessary and the information should be obtained more precisely, This calls for larger samples, but as the sample size increases, each unit of information is obtained at greater cost, The degree of precision may be measured in terms of the standard deviation of the mean, The standard deviation of the mean is inversely proportional to the square root of the sample size, The larger the sample, the smaller the gain in precision by increasing the sample size by one unit.
The nature of the research also has an impact on the sample size, For exploratory research designs, such as those using qualitative research, the sample size is typically small, For conclusive research, such as descriptive surveys, larger samples are required Likewise, if data are being collected on a large number of variables, larger samples are required.
The cumulative effects of sampling error across variables are reduced in a large sample, If sophisticated analysis of the data using multivariate techniques is required, the sample size should be large, The same applies if the data are to be analyzed in great detail, Thus, a larger sample would be required if the data are being analyzed at the subgroup or segment level than if the analysis is limited to the aggregate or total sample, Sample size is influenced by the average size of samples in similar studies. Table 11.2 gives an idea of sample sizes used in different marketing research studies. These sample sizes have been determined based on experience and can serve as rough guidelines, particularly when nonprobability sampling techniques are used.
Finally, the sample size decision should be guided by a consideration of the resource constraints, In any marketing research project, money and time are limited, Other constraints include the availability of qualified personnel for data collection, In the opening duck stamp example, the sample size of 1,000 was determined by resource constraints and the sample size used in similar studies, The sample size required should be adjusted for the incidence of eligible respondents and the completion rate.
Execute the Sampling Process
Execution of the sampling process requires a detailed specification of how the sampling design decisions with respect to the population, sampling frame, sampling unit, sampling technique, and sample size are to be implemented, If households are the sampling unit, an operational definition of a household is needed, Procedures should be specified for vacant housing units and for callbacks in case no one is at home, Detailed information must be provided for all sampling design decisions.
Tourism Department Telephones Birthday Boys and Girls
A telephone survey was conducted for the Florida Department of Tourism to gain an understanding of the travel behavior of in-state residents, In 2008 there were more than 18 million residents in Florida, ranking the state fourth in the United States after California, Texas, and New York, These households were stratified by north, central, and south Florida regions, A computerized random digit sample was used to reach these households. Households were screened to locate family members who met four qualifications:
1. Age 25 or older
2. Live in Florida at least seven months of the year
3. Have lived in Florida for at least two years
4. Have a Florida driver’s license
To obtain a representative sample of qualified individuals, a random method was used to select the respondent from within a household, All household members meeting the four qualifications were listed and the person with the next birthday was selected. Repeated callbacks were made to reach that person, The steps in the sampling design process were as follows.
1. Target population: Adults meeting the four qualifications (element) in a household with a working
telephone number (sampling unit) in the state of Florida (extent) during the survey period (time).
2. Sampling frame: Computer program for generating random telephone numbers
3. Sampling technique: Stratified sampling, The target population was geographically stratified into three regions: north, central, and south Florida.
4. Sample size: 868
5. Execution: Allocate the sample among strata; use computerized random digit dialing; list all the members in the household who meet the four qualifications; select one member of the household using the next birthday method.
An appropriate sampling design process enabled the Florida Department of Tourism to gain valuable insights into the travel behavior of in-state residents, The opening duck stamp ell ample provides another illustration of the sampling design process.
The New York Yankees: Yanking Families to the Game
The New York Yankees is one of America’s favorite baseball teams.
1. As the marketing manager of the New York Yankees, what marketing programs will you design to target families?
2. Identify information at this site that would be useful for marketing research purposes.
3. Search the Internet, as well as your library’s online databases, to obtain information that will assist the marketing manager of the New York Yankees to target families.
4. The New York Yankees want to conduct a telephone survey to determine how to attract more families to the Yankees’ games. Design the sampling process.
5. If the MLB is to conduct Internet surveys to determine how to increase attendance at MLB games, what sampling process would you recommend.