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Applying Market Research to Airport Ground Access 141 interviews and self-completion questionnaires. A personal interview ensures a more thorough com- pletion of questions with the added advantage of enabling the interviewer to query the respondent when answers are ambiguous. The drawbacks of personal interviews are the limited number of surveys that can be completed in a given time frame and the potential for either under-sampling trav- elers who are more time conscious or oversampling travelers who arrive far ahead of scheduled depar- tures (primarily non-business travelers). Personal interviews may be a reasonable option when the survey to be administered is complex. The other approach, self-completion surveys, requires the dis- tribution and collection of questionnaires at designated airport locations such as security checkpoints and aircraft boarding lounges. The advantage of this method is that many surveys can be distributed and completed in a limited amount of time. The drawbacks of self-completion surveys include lim- ited returns from just-in-time air travelers, the inability of the respondent to ask questions about how to properly complete the survey, and the extra effort needed to ensure adequate sample sizes. To collect ground access information from airport employees, surveyors either distribute self- completion questionnaires at the airport work site or mail the forms to home residences. The difficulty in conducting an airport employee survey is the need for a list of all airport employees. The development of this list will be discussed under the topic of sampling frame in the next step. Step 3: Determine the Sampling Frame and Sampling Method The Sampling Frame Obviously, it is not feasible to contact every ground access traveler and ask him or her ques- tions about his or her access trip. In sampling, a small group of ground access travelers is selected to provide information that is considered representative of the entire population of travelers using an airport. The entire population of ground access travelers is known as the universe. The sampling frame is a list representing the universe from which a sample is selected. Because a perfect list is impossible to have, a good research study team will understand the shortcomings of the sampling frame so that it can make allowances in the design of the study. The principal concern in conducting airport market research is the development of the sampling frame. To ensure that results of a survey can be generalized to the entire population of ground access trav- elers, care should be taken to construct a list that is representative of all air travelers and airport employees. The actual implementation of sampling and the selection of a sample are complex tasks and require knowledge of statistics and probability. In developing an airport employee sampling frame, more than one source of information may be required. The airport administration should have a list of companies leasing space from the air- port. Even if this list only provides the names and administrative addresses of the employers, the employers can be contacted and asked to provide more detailed information. This could include information about their different facilities and locations on the airport, the number of employees reporting to each location, and the number of airport-based flight crew employees. Finally, secu- rity access methods used by the airport and air quality ridesharing/trip reduction reports required in certain cities may provide other potential sources of information about airport employees. Probability versus Non-Probability Sampling The heart of sampling is the difference between probability and non-probability sampling. Probability sampling, also referred to as "random sampling," means each sampling unit has an equal, known chance of coming into the sample. In probability sampling, a random sample allows the calculation of the accuracy of the results; non-probability sampling does not. Proba- bility sampling should be used for determining the access choices of air travelers and airport employees because it provides a known degree of accuracy. The degree of accuracy required and the survey sample size is related to (1) the size of the geographic zones that will be used in ana- lyzing the airport ground access market area and (2) the cost of the survey.