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Summary of Recommended Standardized Procedures and Guidelines 39 that occurred during the period of the survey. This may include both positive and negative infor- mation, with respect to the overall survey process. Preserving this information will allow agen- cies to improve on future research projects and proposal submissions because staff writing such documents may consult older examples of these types of documents (Sharp, 2003). 2.7 Assessment of Survey Quality 2.7.1 Q-1: Computing Response Rates Proper calculation of response rates is important because response rates are used by analysts to assess survey quality. Higher response rates are usually desired to reduce the likely incidence of non-response bias. Until recently, CASRO was the only organization with its own method for calculating response rates. However, some years after the development of the CASRO method, the American Association of Public Opinion Research (AAPOR) developed another method for calculating response rates. Both the CASRO and AAPOR formulas are commonly used by sur- vey practitioners. These response rate calculations and related issues are discussed in Section 10.1 of the Technical Appendix. Standardized procedures are proposed regarding the definitions of the components used in the calculation of response rates. Final disposition codes should be divided into four major groups, regardless of the survey modes to be used: 1. Complete interviews; 2. Eligible cases that were not interviewed (non-respondents); 3. Cases of unknown eligibility; and 4. Ineligible cases. These categories can be sub-classified further, depending on the level required by the survey firm and the survey execution method employed. Final disposition codes, adapted from the AAPOR standards, suggested for consistency among transportation surveys, are shown in Table 9. We recommend that the AAPOR (RR3A) formula be adopted for the calculation of response rates for all household and personal travel surveys (Equation 2): SR RR3 A = (2) (SR + PI ) + ( RB + O ) + e A (UH + UO + NC ) where SR = number of complete interviews/questionnaires, PI = number of partial interviews/questionnaires, RB = number of refusals and terminations, O = other, NC = number of non-contacts, UH = unknown if household occupied, UO = unknown other, and eA = estimated proportion of cases of unknown eligibility that are eligible (AAPOR eligibil- ity rate: the same formula for calculating the eligibility rate is used). The eligibility rate for the unknown sample units will vary from survey to survey. It is recom- mended that careful consideration is given to disposition codes, that the bounds of the research are clearly defined, and that the eligibility rate for the unknown sample units should be defined from this analysis. In transport surveys (as recommended as a standard by AAPOR), it is recommended that 1. The estimation of the eligibility rate be left to the discretion of the organization(s) and indi- vidual(s) undertaking the research;