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48 Guidebook for Conducting Airport User Surveys While the effort required to assemble statistical information on the size and characteristics of the airport user population can be considerable, the information has other uses apart from survey design. Combining information on airport user characteristics with the survey data will produce a more integrated profile of airport activity. 4.2 Sampling Strategy and Plan 4.2.1 Determining Strategy After identifying the population to be surveyed and determining whether this population can be counted, the next step in the survey design is to determine whether a census or sample sur- vey should be used. Depending on the type of survey the following strategies and plans must be determined: If a census survey is to be used, determine how each individual in the population will be contacted. If a sample survey is to be used, determine: The sampling method (i.e., random, clustering, stratification, etc.), taking into account the overall budget and the feasibility and efficiency of collecting data using each method. The sampling plan (i.e., multi-stage sampling, choice of strata or clusters, methods of sam- pling within clusters). The sampling plan to be used is very dependent on the type of survey being conducted (see Chapters 5 through 10). The sample size for the level of accuracy required. Whether a census or sample survey is used, steps for minimizing non-responses should be identified. Consideration should also be given to whether further analysis of individuals not responding is required and overall estimates adjusted. The sampling plan to be used is very dependent on the survey type being conducted and is covered in the chapter for each survey type (for example, see Section 5.3 for surveys of air passengers). 4.2.2 Minimizing Bias The two main sources of bias in airport user surveys and measures to reduce their occurrence follow: Use of non-random sampling procedures. Bias can be reduced by the following measures: Selecting an appropriate sampling method for the type of survey population, location, and time constraints. Training interviewers in sampling techniques. Supervising the interviewers. Developing backup plans for unexpected events affecting the sampling process, such as delayed or cancelled flights. Non-response of sampled individuals, as non-respondents can have significantly different characteristics from those responding. Response rates (also discussed in Section 4.8) can be improved by the following measures: Choosing the survey location and time so that respondents have time available to respond. Keeping the questionnaire short, as well as easy to understand and complete. Using well-trained, experienced, and friendly staff to conduct interviews. Using multilingual interviewer staff and questionnaires if appropriate. Providing incentives, such as pens or coupons for free coffee. Incentives are generally not required for airport user surveys but can help if respondents are significantly inconvenienced.