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Tenant Surveys 127 the required information and may be busy with customers, so interviews should be done by appoint- ment. Tenant staff may also have to check company records to respond to some questions, thus some questions might go unanswered with only a single interview. Generally, tenants are contacted by mail or telephone to advise them about the survey and possibly schedule an appointment. Send- ing the questionnaire in advance gives the tenants a chance to assemble any information needed. The advantage of on-site surveys is that a deeper understanding of the issues is possible through in-person interviews. The major disadvantages are that scheduling all the required interviews is often difficult and on-site surveys are more costly to conduct. 7.2.4 Confidentiality As confidentiality is an issue for tenant surveys, it is recommended that they be conducted by respected third parties. Assurances should be given to the tenants that the information will be kept in strict confidence and provided to the airport operator in aggregate form only. 7.3 Sampling Methodology In the case of Internet or mail surveys, questionnaires or survey requests will generally be sent to all tenants at the airport. However, including all tenants could be costly, especially for on-site interviews at large airports. If only a sample of tenants is required, a stratified sample of tenants should be surveyed where the tenants are grouped by category--for example, concessions, air- lines, government and security agencies, individual aircraft owners, and other organizations. The sample size will depend on the purpose of the survey, the desired level of accuracy, and the num- ber of tenants in each category. For random sampling, required sample sizes to achieve an accuracy of better than 5 percent- age points for a categorical variable are given in Table 7-1 for a range of total numbers of tenants at the airport. As discussed in Section 3.4, the required sample sizes vary depending on the propor- tion 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, experi- ence of other airports, and/or knowledge of the airport. Required sample sizes are given for three values of this proportion: 0.5, 0.25 and 0.1.29 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 propor- tion of 0.5 should be used unless the survey sponsor is primarily interested in questions where the Table 7-1. Sample sizes required for accuracy of better than 5 percentage points in a categorical variable. Total Number of Sample Size Required for Category Proportion: Tenants at Airport p = 0.5 p = 0.25 p = 0.1 30 28 27 25 50 44 43 37 75 63 60 49 100 80 74 58 200 132 118 82 400 196 168 103 Note: Assumes random sampling. 29For 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.