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44 Guidebook for Conducting Airport User Surveys weights will vary depending on the survey type. For example, for surveys of passengers exiting the security checkpoint, weights for surveys collected in a particular hour could be set equal to the total numbers of passengers going through security in that hour divided by the numbers of surveys collected in that hour. The numbers of passengers exiting the security in each hour of the survey could possibly be obtained from the security authority or counted manually. Rather than applying equal weights to all passengers, or to all passengers within a group, weights can be applied so that the sample is more representative of the population. For example, weights could be set so that the distribution of surveyed passengers by airline matched the actual distribution of passengers by airline during the survey period. If the actual numbers are not avail- able from the airlines, they could be estimated based on the seat capacity of departing flights and average load factors for each airline during that month. In some cases, different sets of weights may be required for analyzing different characteristics of the population. For example, in a survey of passengers, questions relating to their travel to the airport and air trip are relevant to the respondent's companion or companions, as well as the respondent, but personal questions such as gender and age apply only to the respondent. By including questions on the number of travelers in the group and number of questionnaires com- pleted by others in the group, it is possible to define two sets of weights, one for the airport access and trip characteristics and the other for personal traveler characteristics. Although the air travel party and those traveling together to the airport are generally the same, this is not always the case, as discussed in Section 5.2. People who have been attending an event such as a business meeting or conference may travel together to the airport but then take differ- ent flights, while others may travel separately to the airport and meet there to travel together on the same flight. The latter situation is particularly common with large air travel parties such as school groups or sports teams. Therefore, it is desirable for passenger surveys to ask how many people are traveling together on the same flight as well as how many people traveled to the air- port together with the respondent. This specificity will allow separate weights to be calculated for the air travel party characteristics and the ground access travel characteristics. 3.6 Summary The goal of this chapter has been to provide the non-statistician with an understanding of the basic statistical principles behind sample design, the terminology involved, and the importance of considering how the sampling approach and the sample size interact to determine the statisti- cal accuracy of the resulting data. Although it is not unusual to present the results of airport user surveys without any real discussion of the likely accuracy of those results, it is not good practice. If the results are to be used for decision making, it is the responsibility of those managing the sur- vey process to decide how accurate the results need to be, design the survey accordingly, and ensure that decision makers using the survey results are aware of the likely accuracy of the results. One of the most fundamental questions in planning a survey is deciding how large a sample size is required, because this has a major influence on the cost of conducting the survey. As discussed in this chapter, the decision on sample size is in turn influenced by the sampling approach adopted, which also affects the cost of conducting the survey. An appreciation of the statistical basis for assessing the likely accuracy of the results of a particular survey approach and the sample size required to achieve a desired level of accuracy is therefore critical to effective survey planning.