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128 Guidebook for Conducting Airport User Surveys proportion is lower. Further examples for determining the required sample size are provided in Appendix B. Depending on the purpose of the survey, it may be appropriate to sample a greater proportion of some subgroups than others. If a similar level of accuracy is desired for each subgroup, a higher sampling fraction would be used for smaller subgroups or for subgroups where the variance of the characteristic of interest, say economic impact, is greatest. Thus, subgroups with a high economic impact such as airlines would have a higher sampling fraction than owners of individual private aircraft. For economic impact surveys, the treatment of non-responses can significantly affect the results. The economic impact of individual tenants varies greatly, particularly between groups of tenants. Use of only the data from responding tenants would lead to underestimation of the total impact, and expanding the sample results on the basis of the average response for each group of tenants could result in large errors, depending on the tenants that did not respond. Every effort should be made to obtain some response from all tenants, even if only basic information--such as the num- ber of employees or gross revenues--is obtained from some organizations. This information can then be used to develop weights to expand the data received from each group of tenants to repre- sent the total group. If no information can be obtained from some tenants, the airport will always know something about the size of each tenant. In many cases they will know the number of employ- ees from issuing security badges. They will also know the size of the area leased and in many cases will have traffic or revenue data from required reports or concession fees. This information can be used to estimate the relevant economic measures for the non-respondents, based on the data from the responding tenants. Mystery shopper surveys will generally be conducted on all food and beverage and retail con- cessionaire tenants (see Section 7.7). 7.4 Questionnaire Wording and Length Tenant feedback and economic impact surveys should take less than 20 minutes, preferably much less, including finding the data. There is always a temptation to ask for more detailed infor- mation than is required; however, this can significantly affect the response rate. The survey should be restricted to requests for information that a knowledgeable person within the orga- nization will know immediately or be able to obtain easily. As discussed in Section 4.3, deter- mine in advance exactly what information is required and how the response from each question is going to be used. A sample tenant questionnaire is provided in Appendix I. 7.5 Measures to Obtain Adequate Response A high response rate is important for obtaining accurate, unbiased results. The following measures could be taken to improve the response rate: Include the name of the company or organization conducting the survey and the survey sponsor in the introduction to the survey. The introduction should clearly state the purpose and how the results will be used, highlighting aspects that could benefit the respondents as airport tenants. Make it easy for the tenants to respond. Use of email to distribute the questionnaire allows it to be easily forwarded to the appropriate person. Try to keep the requested data to things the respondent will know immediately or be able to find out easily.