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Planning a Survey 13 expected. An example would be asking air passengers "Where did you begin your trip to the airport today?" with the goal of obtaining trip origin location information for airport access trips. This question is ambiguous for air travelers who are surveyed on the return leg of a one-day round trip. Typically, a survey's purpose will be articulated using goals and objectives. Goals are broad, general statements of the information the survey is intended to obtain; objectives are more spe- cific. For example, the goal might be to gather information on air passenger ground transporta- tion use; the objectives might be to determine where passengers began their trip to the airport, which ground access modes they used, how they got to those modes, and why they chose the par- ticular modes they used. Well-defined goals and objectives will provide the necessary guidance for the development of useful and appropriately worded questions. 2.1.2 The Planners A strong leader or project manager is needed to clearly define the purpose of a potential sur- vey. Planning by committee has a tendency to be time consuming and awkward. Nonetheless, the project manager may need to involve a few key people. Who should be involved in the planning process? The following groups should at least be considered: Decision makers--those who are expected to make the decisions or take the actions the survey is supposed to address. Data users--those who are expected to use the data to support the decision makers in plan- ning, modeling or marketing. Potential partners--organizations that might share an interest in the results and might there- fore provide support or even financial assistance. Potential duplicators--those who might be conducting the same or similar research at the same time and thus get in the way. Having two surveys conducted at the same airport at the same time by different parties is generally not a good idea. This is not to say that all of these groups need to be involved. It is important, however, at least to consider each of them and their potential either to help or to hinder the survey effort. In addition, two distinct teams will participate in the survey: The survey planning team will be responsible for the planning and design of the survey (refer to Section 2.6). The survey implementation team will conduct the survey and will consist of the interviewers, supervisors, and support personnel (refer to Section 4.5.2). Some of its members will also serve on the survey planning team. 2.2 Selecting the Survey Method A key step in planning a survey is choosing the appropriate survey method. The survey meth- ods discussed in this guidebook include passenger intercept, mail, telephone, and Internet. Choosing the appropriate method for any survey involves consideration of a wide variety of fac- tors. Perhaps the key tradeoff, however, is between cost and quality. As a general rule, the higher the quality desired, the more expensive the survey will be. Quality in this case includes the following: Data quality--Will the questions be understood and elicit the desired information? Will all of the questions be answered? Will the answers be accurate? Will answers to any open-ended questions be clear and complete?

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14 Guidebook for Conducting Airport User Surveys Response rate--Will the survey methodology achieve a high enough response rate (the ratio of those responding to those approached to participate in the survey) to be able to generalize the results to the population of interest? It is clear from the literature that respondents often differ from non-respondents in material ways (Dillman, 2000; Groves and Lyberg, 1988; Lessler and Kalsbeek, 1992; Montaquila et al., 2007), and of course the survey team cannot know much about those who do not respond. Although experts differ on this point, a general rule of thumb is that the response rate needs to be at least 50% for a researcher to be reasonably confident that the results are representative. How- ever, lack of response bias (difference in the mean value of the characteristics of interest between respondents and the population being surveyed) is more important than a high response rate (Bab- bie, 1973). Regardless of what is considered an acceptable response rate, the lower the response rate, the more caution must be used in interpreting the data. Response rates vary widely by survey method and are generally fairly high for interview surveys of air passengers, but much lower for surveys conducted by mail or telephone. The survey methods and their advantages and disadvantages are outlined below; more details can be found in the chapters devoted to the specific types of surveys. Intercept surveys, generally undertaken with passengers, are performed by approaching poten- tial respondents as they pass a particular location, such as when they enter the terminal building or leave security screening. They are usually performed as an interview survey, in which survey staff ask the questions and record the responses. Interview surveys can also be performed by selecting respondents in a particular location, such as an airline gate lounge, using a defined sampling rule to identify who to approach. The main advantage of interview surveys is the potential for a high response rate and high data quality resulting from the use of professional interviewers. In addition, sample control can be maximized using this approach. The primary disadvantage of interview sur- veys is their cost, which can be substantial. Self-completed surveys that are handed out, completed by the respondent, then returned, either in person or by mail, are frequently used with air passenger, employee and tenant surveys. The key advantage of this approach is its relatively low cost, as one interviewer can hand out a large num- ber of questionnaires in a given time period. Disadvantages of this approach include lower response rates and inferior data quality. Length and complexity are also concerns; generally, airports try to keep such surveys short and simple to maximize the number of responses and completeness of the information they get back. Mail surveys are infrequently used by airports but could be useful for tenants who are spread out across the airport's premises. Their main advantage is their relatively low cost. Key disadvantages are similar to those of surveys that are handed out: low response rates, inferior data quality, and the need to keep the survey short and self-explanatory. Telephone surveys are useful for surveying households and businesses in the area served by the airport, but are not a practical way to survey air passengers because there is usually no way to obtain a list of telephone numbers of a representative sample of air passengers. The key advantages of this method are the ability to obtain a representative sample of a large and dispersed target population, such as area residents or airport employees, and a reasonable level of quality control due to the use of professional interviewers. Disadvantages include their moderately high cost and the need to allow a long enough survey period for the call center to maximize the response rate and the repre- sentativeness of the sample (see Section 7.3). Internet surveys have become increasingly popular in recent years for some types of surveys, partly because they are extremely inexpensive. They are also easy to program and deploy with

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Planning a Survey 15 software that is readily available and inexpensive or even free. This software allows researchers to upload questionnaires to the World Wide Web (Web) and send out email invitations to par- ticipate. Responses are entered directly on the Web and stored in a database by the software. Branching to only appropriate questions and control of valid responses are possible. Tabulated responses and simple graphics are usually created by the software, and the data can be down- loaded for more detailed analysis. Often respondents can save partially completed responses and return later to finish them, which can increase the response rate. Some programs also allow researchers to track respondents and non-respondents so that reminders can be sent only to those who have not yet responded. However, it is important that the survey include a large enough sample to be representative. Internet surveys have three main disadvantages. First, not everyone has access to the Internet, particularly in their homes, but also in the workplace. (Consider the workplace example of an airport: office staff usually have Internet access at their desks but other types of staff, such as ramp personnel, probably do not have Internet access at all.) Second, response rates to Internet sur- veys are generally the lowest of any type of survey, reducing the likelihood of obtaining a repre- sentative result. Third, Internet surveys have to be fairly short and simple. The reasons for this include different technological capabilities among respondents, the absence of anyone to clarify issues the respondent may have, and perhaps people's general expectation that online transac- tions should move rapidly. Other issues that should be considered when selecting the survey method include the following: Speed: How quickly does research need to get done? As examples, mail surveys take a long time to come back; surveys that are handed out to passengers are returned almost immedi- ately if they are returned at all. Complexity: How complicated are the inquiries? The more complicated the questions, the more important it becomes to have a person available to clarify issues the respondent may have and provide assistance, as with an intercept interview. Flow control: Does the order in which questions are asked need to be controlled? If so, then certain methods, such as handouts and mail, are excluded. An example of the need to control flow might be asking what airports in the area come to mind, then following up by naming and inquiring about all airports not mentioned; the second question in effect "gives away" the possible answers to the first. Visual aids: Are visual aids needed? An example might be a stated preference survey on the likely use of a proposed new ground access mode, in which respondents are shown images of the proposed mode in order to ensure that they have a clear understanding of what the mode would be like to use. Methods such as telephone surveys that do not permit the display of such aids would be excluded. Confidentiality: How important is confidentiality to the study? In many cases, airport surveys are not asking particularly sensitive questions and confidentiality is not a major issue. Passen- ger surveys, however, frequently ask about the origin of the trip to the airport, which often means home addresses. In this case, confidentiality is obviously of high importance. It bears remembering that in an age of identity theft and widespread consumer fraud, concerns about confidentiality are legitimate. Some survey methods are more confidential than others. Mail surveys can be designed with no identifying information on the survey response form so that respondents can remain anonymous if they wish; in-person interviews and Internet surveys perhaps raise the most concerns about confidentiality, but the latter can be made quite secure with the proper hard- ware and software. The main advantages and disadvantages and each survey method are summarized in Table 2-1.