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136 Guidebook for Conducting Airport User Surveys on the fly, they will come up with many different wordings, which will cause inconsistencies in the way questions are interpreted and possibly the responses obtained. This occurrence can compro- mise the utility of the results. 8.6 Survey Budget A number of factors influence a telephone survey budget. In terms of the survey itself, inter- view length, types of questions (open-ended versus closed-ended), languages to be used (both the number of languages to be used and the difficulty of recruiting interviewers who are fluent), call sequences, the level of data analysis required, and the nature of desired deliverables all play impor- tant roles. Geography is also important, in two respects. In areas with a high cost of living, survey costs will be higher because rents, salaries, and wages are higher. And in urban areas, costs will be higher because cooperation is more difficult to achieve than it is in rural or suburban areas. Given these factors, it is difficult to give "a price" for a telephone survey. However, if one con- siders a typical survey, which is 10 minutes long, directed to the general public, conducted in a moderately cooperative market, and includes two open-ended questions, the unit price (cost per interview) is likely to be in the $40 to $50 range. Surveys that are shorter, are targeted to more coop- erative areas, or have fewer open-ended questions will generally cost less; those that have a nar- rower target audience (e.g., only people who have made an air trip in the preceding year) will tend to cost more. 8.7 Summary Most surveys of area residents are conducted to obtain information for marketing and airport planning purposes. Typically, these surveys are conducted by telephone. Unfortunately, there are no lists of telephone numbers for all members of the general public from which an airport could select a list of people to call. The widely accepted alternative approach is to use RDD. Perhaps the greatest source of bias in telephone surveys is the refusal of potential interviewees to participate. Smaller sources of bias are non-telephone households and telephone numbers that are not answered after repeated calls. A more recent, and growing, cause for concern is house- holds that have only cellular telephones. Most RDD samples exclude cell phones because it is ille- gal to dial cell phone numbers automatically and because the respondent must pay the cost of the call. Experience suggests that surveys of the general public will start to experience unacceptable rates of refusals and terminations when the interview exceeds about 10 minutes. Cooperation rates are highest when interviews are five minutes or less. The cost for a telephone survey is influenced by a wide variety of factors--including interview length, types of questions, languages to be used, how many calls are placed to each number, the level of data analysis required, and the nature of desired deliverables--making it difficult to pro- vide a generalized cost estimate. If one considers a typical 10-minute survey, however, the unit price (cost per interview) is likely to be in the $40 to $50 range.