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122 Guidebook for Conducting Airport User Surveys Table 6-1. Sample sizes required for accuracy of better than 5 percentage points in a categorical variable. Total Number of Sample Size Required for Category Proportion: Employees at Airport p = 0.5 p = 0.25 p = 0.1 50 44 43 37 100 80 74 58 200 132 118 82 500 218 183 108 1,000 280 225 122 5,000 360 272 135 Note: Assumes random sampling. For on-site surveys, the number of responses to be collected should be determined in advance. (See Chapter 3 for a discussion on the different sampling methods and required sample sizes.) An estimate of the number of employees at the airport will be required, even if this is only approximate. For random sampling, required sample sizes to achieve an accuracy of better than ±5 percentage points27 for a categorical variable are given in Table 6-1 for a range of total num- bers of employees at the airport. As discussed in Section 3.4, the required sample sizes vary depending on the proportion of the population in the category of interest. This proportion will not be known at the time of planning the survey and must be estimated, at least approximately, based on past surveys, experience of other airports, and knowledge of the airport. Required sam- ple sizes are given for three values of this proportion: 0.5, 0.25, and 0.1.28 The largest samples are required when the proportion of the population in the category of interest is 0.5. Because most surveys ask multiple questions with various unknown proportions in each category, the sample size corresponding to a proportion of 0.5 should be used unless the survey sponsor is primarily interested in questions where the proportion is lower. Further examples for determining the required sample size are provided in Appendix B. Note that for categorical questions with a fairly low proportion of respondents in a category of interest, a margin of error of ±5 percentage points may not be considered accurate enough. On-site surveys should cover weekday and weekend periods and morning, afternoon, and evening shifts as well as a wide range of locations. The locations should include areas where the employee groups go during their breaks, such as break rooms and public food courts. Given the ad hoc nature of selecting employees to interview, the resulting sample may not be truly representative of the employee population. It is therefore recommended that responses be weighted to match subgroup sizes, for example by employer category, shift schedules, or work location. 6.4 Questionnaire Wording and Length Questionnaire length, format, and clarity, and the use of pre-tests and pilot tests, are discussed in Chapter 4. The questionnaire for employee surveys should be relatively short, as employees are usually busy during work time and value their breaks. The time needed to complete the survey should be no more than 5 to 10 minutes. 27 As discussed in Section 3.2, accuracy to within 5 percentage points is very different and usually much less stringent than accuracy to within 5% of the proportion in the category. 28 For proportions (p) greater than 0.5, the required sample size is the same as for the proportion 1-p. For example, for a proportion p = 0.75, the required sample size is the same as for p = 0.25.