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CHAPTER 4 Survey Design This chapter describes the process of designing a survey after the initial steps and planning deci- sions that were discussed in Chapter 2 have been completed. Each part of the design process needs to be given careful consideration, because the way in which each is addressed will affect the qual- ity of the results as well as the costs of performing the survey. Considerations that are specific to particular types of airport-user surveys are discussed in more detail in subsequent chapters, which should therefore be read in conjunction with this chapter in planning a survey of a particular type. 4.1 Survey Population The results of a survey represent a sample drawn from the larger population, although in some cases the sample may consist of the entire population (often referred to as a 100% sample or a census survey). This section addresses the need to clearly define the population of interest, including the characteristics of the population required for survey planning and where this infor- mation can be obtained. 4.1.1 Defining the Population of Interest The first step in any survey design is to define the target population for which information is to be collected. For example, the target population could be origin/destination passengers at the airport, in which case connecting passengers would be excluded, or all airline or airport-based employees using the airport terminal, in which case flight crews not based at the airport would be included. While the target population may seem self-evident at first, the exact definition will influence the survey methodology and sampling strategy and requires careful thought. The population available to be surveyed may differ from the desired target population, depending on the survey period and method used to conduct the survey. For example, the sur- vey sponsor may wish to obtain information on all air passengers using the airport throughout the year. Performing a survey over a relatively short period limits the available population to trav- elers during that period, whose characteristics may differ from those at other times of the year. Similarly, performing an Internet survey of airport employees limits the available population to those with Internet access. The limitations imposed when a survey is performed must be fully understood when inter- preting the results. When the characteristics of the target population change throughout the year, as they generally do for air passengers, there is no way to know whether a survey performed over a fairly short period will provide a reasonable representation of average annual conditions. If information on average annual conditions is desired, it will be necessary to perform the survey over a number of different periods throughout the year to account for seasonal variations. 45
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46 Guidebook for Conducting Airport User Surveys In some cases, the individual members of the target population can be identified prior to the survey. For example, it will generally be possible to obtain a list of all airport tenants and it may be possible to obtain a list of all airport-based employees. However, it will not be possible to iden- tify every air passenger using the airport during a specific period. In the case of area residents or businesses, while it may be possible to define the entire population, in practice it will usually not be feasible to assemble a comprehensive list. In cases where it is not possible to identify individual members of the population prior to the survey, it is often possible to obtain some information about the relevant characteristics of the population, although this information will not necessarily be organized in a readily usable for- mat and may require some additional research. Some of this information may be prospective, such as flight arrival and departure times and the aircraft types assigned to each flight that can be obtained from the Official Airline Guide or other sources of flight schedule information. Other information may be historical and require extrapolation to the period of the survey. For example, fairly detailed information on air passenger trips in the United States by airline and market is available from the Bureau of Transportation Statistics of the U.S. Department of Transportation, and there are generally similar sources of information in other countries. Design of an appropriate survey sampling plan (discussed in Section 4.2) requires a well- defined target population, as well as information on the size of the target population. The types of data that can be used to determine the size of the target population include the following: · Enplaned/deplaned air passengers for the period of the survey. · Aircraft movements by time of day and seat capacity, typically available from airport flight information systems, gate assignment systems or tower records. · Vehicle counts on airport roadways, typically collected using automatic traffic counters. · Parking exit counts and duration data from parking systems. · Curb activity counts. The above list is not exhaustive. The exact data required to determine the size of the target population for a survey will be a function of the goal of the survey, the target population, and the type of survey being conducted. Additional details on defining survey populations are included in the chapters devoted to each survey type. 4.1.2 Identifying Sources of Information In addition to information on the overall size of the survey population, assembly of informa- tion on the composition and characteristics of the population will generally be desirable. This information will enable development of a more detailed sampling plan, as well as provide an indi- cation of the extent to which the survey results correspond to the distribution of characteristics within the population. This in turn will allow appropriate weights to be assigned to the individual survey responses so that the survey results can be extrapolated to the population as a whole. In some cases, detailed data on the size or characteristics of the population may not be readily available. For example, an airport authority may not have traffic counts on the terminal roadways or may have no information on the number of air parties arriving at the airport by shared-ride van or using off-airport parking. It is increasingly common for airports to use auto- mated vehicle identification (AVI) systems to track the number of trips made by different classes of commercial vehicles, such as shared-ride vans or hotel shuttles. However, even in these cases information on the occupancy of those vehicles is generally not available. Smaller airports may have no idea how many such trips are made. Similarly, where airport employers provide park- ing for their employees, the airport operator may have no information on the number of park- ing permits that have been issued, much less the number of vehicles parked at the airport by those
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Survey Design 47 employees on a given day. While an airport operator will generally know how many parking per- mits it has issued to its own employees, it may not know how often they are used. Therefore, in planning a survey, one of the first steps is to decide what characteristics of the population are desirable to know in order to develop appropriate weighting factors and what data on these characteristics are already available. Where information on desired characteristics is not readily available, what would be involved in obtaining it? Answering this question may require some consultation with operational staff or external agencies. As it becomes clearer how much work would be required, a decision can be made whether the benefits of improving the survey weighting process justify the effort involved. 4.1.3 Determining Population Characteristics Although the total number of air passengers per month is generally known from airline reports and the total number of airport employees is known from employment records, the more rele- vant information is how the flow of passengers or employees varies by time of day or day of the week. Because the number of surveyed passengers in any hour or the work-shift patterns of sur- veyed employees may not correspond to the overall distribution of activity, it is desirable to know how passenger flows or employee shifts vary over the day and week. Some airports require airlines to report air passenger statistics for each flight. In this case it is fairly straightforward to assemble data on passenger flows by time of day, although adjustments will need to be made for connecting passengers, unless these are reported separately. In the more common situation, where airlines do not report passenger traffic at this level of detail, one approach is to analyze the distribution of seats on departing and arriving flights and make assumptions about load factors. Where passenger data at the level of individual flights are not routinely reported, it may be possible to obtain this level of information for the period of the survey from the airline departure desk or by counting boarding passengers. Data may also be available from the Transportation Security Administration on the hourly variation in the flow of people through the security screening checkpoints, which can be used to refine assumptions about the variation in airline load factors at different times during the week. Similarly, it may be possible to obtain information from airport employers on the number of their employees who will start their shift at different times throughout the week for the period of the survey. In the case of airport ground access modes used by air passengers and airport employees, it would be helpful to know how the use of the different modes varies over the week. In principle, detailed information on the number of parking exits by time, together with how long those vehi- cles have been parked, should be available from the parking revenue control system. However, at many airports this information is not readily extracted from the database, and some manual analy- sis of printouts or even a sample of parking tickets may be necessary. Information on the use of other modes, particularly drop-off and pick-up by private vehicle, is more difficult to obtain. Counts of vehicle trips may be available from AVI systems or trip fees paid by the operators, but these counts will not necessarily correspond to air passenger use, because of variation in air party size, more than one air party in a vehicle, and vehicles traveling to or from the airport without passengers (deadheading). It may be necessary to perform an occupancy survey to determine appropriate assumptions for the variation in vehicle occupancy over the week. These counts can be supplemented by installing traffic counters at strategic locations on the airport roadway during the period of the survey. In some cases, the survey sponsor may not even know the overall size of the target population. For example, if the target population is local area businesses, the total number may not be known to the airport operator. Some research may be necessary to obtain data from external sources, such as city and county business license records.