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Survey Design 69 the city name wrong, is much more time-consuming. The more redundant information that is obtained in the survey, such as asking for both the zip code and state (or province) of a respondent's home, the easier it is to identify and correct any errors. "Self-completed Surveys" in Section 5.2.2 includes a discussion on the verification of air party size that may be helpful in understanding what can be involved in the data verification and cleaning process. Data editing is the final step in the process and involves making the necessary changes to the data file to correct any known errors. It is good practice to preserve the originally reported data in one set of data file variables (these may be referred to as fields or columns) and copy the data to a different set of variables before editing it. This procedure allows the data that have been changed to be identified later. Then changes can be revised--or even reversed--if subsequent information comes to light. 4.10.2 Benefits of Using Interviewers for Data Entry Where the survey takes place over a relatively long period, such as several weeks, it is highly desirable to start on the data entry and verification process while the survey is still in progress. This can help identify any problems with the survey questions or procedures while there is still some opportunity to correct them during the remainder of the survey. When the survey schedule permits, it may be advantageous to have contract staff hired as interviewers spend some time assisting with the data entry task. Exposing them to the difficul- ties involved in transcribing the information from the survey forms might lead them to take more care with the way they record responses during the remainder of the survey. Also, because extra interviewers are usually required as backup, using them for data entry helps keep them pro- ductively employed when they are not required for interviewing. In general, however, temporary staff hired as interviewers will not have the data entry skills to efficiently perform the majority of this work. In addition, they will generally not have enough local knowledge about the airport and ground transportation system to perform effective veri- fication and error correction. Therefore verification and error correction will require the active involvement of the survey planning team and possibly the assistance of other operations or plan- ning staff with the necessary local knowledge. 4.11 Analysis and Reporting of Survey Results At this stage of the project the survey data collection is complete, the data have been entered into appropriate databases, and the data have been checked for the internal integrity of each response and cleaned accordingly. It is now time to undertake the analysis, as determined by the goals of the survey project. 4.11.1 Considerations for Doing the Analysis In-House When the analysis will be completed by in-house staff, the resources necessary may not need to be included as part of the survey project budget. It is appropriate to use in-house resources if the following considerations make it feasible: There is sufficient expertise in-house to complete the tasks listed in this section, and this expertise is available in a timely fashion after the data collection period. The appropriate software and hardware exists to complete the analysis. Modern spreadsheet applications have many data analysis capabilities. The requirements may extend beyond these capabilities, in which case the survey sponsor should consider using the more powerful statis- tical features of applications such as SAS or SPSS.

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70 Guidebook for Conducting Airport User Surveys 4.11.2 Weighting and Expansion Once the data have been cleaned and made available to the analyst, one of the first priorities is the assignment of one or more weighting factors to each interview response. In the case of sam- ple surveys that have been conducted according to a strict sampling plan, the response weights will be determined as part of this plan, as discussed in Sections 3.5 and 5.5. These weights can be added to each interview record. In the case of groundside surveys, discussed in Section 5.9, there will be one weight assigned to the interview record based on the vehicle count at the facility and a second weight assigned based on the passenger volumes. The analysis may utilize either of these weight factors depend- ing on the focus of the results, passenger versus vehicle. Depending on the available data, these census counts might include the following: Air passenger counts, possibly by hour or by day. Parking ticket counts. Automated vehicle identification system counts for modes other than private vehicles. Roadway traffic counts. Control count observation surveys--such as at the curb area, security clearance area, or other suitable location--for determining the effective sampling rate and weighting of responses to be used in the analysis. 4.11.3 Tabulation and Interpretation Tabulation and interpretation are the core of the analytical process. Typically, this process begins with basic frequency counts of the key variables and cross-tabulations. This step can often reveal unexpected relationships as well as unexpected problems in the data. It is recommended to do basic frequency counts on all variables to get an initial look at the numbers of valid responses for each question. The process of data analysis is determined by the goals of the survey and is often a personally defined process, unique to the analyst. Invariably the analysis will reveal many interesting results that appear to warrant further analytical effort. However, further analysis should only be pur- sued as time and resources permit. It is in these secondary analyses that the additional value of the data will be exploited. 4.11.4 Survey Accuracy and Limitations An important, but sometimes overlooked, purpose of the data analyst is to identify the accu- racy of the results as well as the limitations within the survey data. Where results are provided for subgroups of the population, the accuracy of the estimates for these subgroups should also be identified. In some cases The sample size may not be sufficient, and consequently it may not be feasible to drill down to increasing levels of detail. The conduct of the survey or response rates will produce limitations in how the data can be analyzed or the level of detail that can be achieved in the analysis. Avoiding these limitations is a key reason to include the expertise of a data analyst from the outset of the survey project. The limitations may be a function of the questions, how they are coded, and what responses were obtained. These limitations may not be revealed using data analysis techniques alone. 4.11.5 Report Preparation and Presentations The primary reports and presentations must be directed towards the goals and purpose of the survey. A formal report is generally required and will be a substantive document that fulfills the