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Procedures and Measures for Further Research 67 demographics, from households that were more difficult to contact. Although this research was able to confirm characteristics of non-respondents found in other work, it was not possible to draw any definitive conclusions about how many refusals/non-contacts should be converted for every call attempt to reduce the overall incidence of bias in data set. It is recommended this issue be examined in greater depth in the future. It is suggested that multiple call-history files be ana- lyzed as part of any future research effort. One of the main difficulties in comparing different call history files is that disposition codes are inconsistently defined among travel surveys. In light of this, it is suggested that future analysis should use files from contemporary surveys that are able to adopt the definitions proposed in this project. 4.3.8 Effect of Interview Mode on Recruitment and Non-Response Rates The effect of interview mode on recruitment and non-response rates is related to the section on personalized interview techniques, Section 4.3.3, except that the focus is different. In this case, the issue is whether different modes of survey will have different impacts on recruitment rates and on eventual non-response rates. The same experiment probably could be conducted for this as would be envisaged for Section 4.3.3. However, the difference in this case will be that the focus is on whether different interview modes used in recruitment are associated with signifi- cantly different recruitment rates and what effect the different modes have on actual completion rates for the survey. 4.3.9 Unknown Eligibility Rates In defining standardized procedures for computing response rates, the issue of the estimated rate of eligibility for those contacts that remained with unknown eligibility was recommended as being left to the survey firm. However, better guidance would be preferred for this issue because it has a critical impact on the calculation of response rates. Effectively, this requires the acquisi- tion of a number of additional call-history files from which analysis can be conducted on the eli- gibility rates at different points in the calling. Ideally, these files should be obtained from surveys that have used 10 or more calls as the limit for trying to recruit households so that it is possible to determine an eligibility rate for a 5-call limit from information obtained from calls made beyond the fifth attempt. 4.3.10 Data Archiving in Transportation In this report, we have proposed standardized procedures for data archiving for household travel surveys (see Section 2.6.4). However, past transportation surveys have not been archived according to the standards. The research that is needed is to archive data, using the standardized procedures, and then test the usefulness and effectiveness of the archiving. This may then result in modifications to the proposed procedures.