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66 Standardized Procedures for Personal Travel Surveys would need to be called and also in relation to the number of pre-notification letters that would need to be mailed out. However, it would also be useful to know the characteristics of households that respond positively to recruitment calls after subscribing to the registry and whether their character- istics differ from the characteristics of households that respond negatively to survey recruitment calls. This will give an understanding of the non-response bias and is important to account for in household travel survey results. A possibility is to determine whether it is possible to obtain a list of households subscribed to the registry and then to compare response rates, characteristics, etc., among households recruited that are on the registry and those that are not. 4.3.6 Initial Contacts Initial contacts are discussed in Section 2.2.7 of this report and Section 5.7 of the Technical Appendix. However, due to limited information, standardized procedures and guidelines could not be suggested. Thus, further research is required that investigates the phrasing of recruitment scripts and other contact materials to enable the development of a suggested consistent approach for the wordings of such materials. This will also depend on the nature of the survey and client requirements. Again, the method that would be preferred is to test several different alternatives in a side-by- side comparison in actual surveys in more than one location. The goal would be to compare refusal and termination rates according to the alternative methods of initial contact, including the effects of pre-notification letters, and alternative ways of phrasing the opening of the recruitment script. 4.3.7 Refusal and Non-Contact Conversions It has been well documented that response rates have been declining and that it is becoming increasingly difficult to get households and individuals to agree to participate in travel surveys. Among other things, this may be attributed increasingly to lengthy and complex surveys (increased respondent burden), more physical barriers inhibiting contact with prospective participants such as call-screening devices (telephone surveys), and gated communities (face-to-face surveys) (Kalfs and van Evert, 2003; Kam and Morris, 1999; Melevin et al., 1998; Oldendick and Link, 1999; Vogt And Stewart, 2001). Also, increasing numbers of marketing surveys have led people to perceive increased respondent burden; therefore, these individuals no longer even consider participating (Black and Safir, 2000; Kalfs and van Evert, 2003). There are two broad categories for unit non-response: refusals (hard refusals, soft refusals, and terminations) and non-contacts (busy, no reply, and answering machines). Unit non-response becomes problematic if the responses of refusers and non-contacts differ significantly from the responses of contacts because this will add to non-response bias (Zmud, 2003). For example, it has been found that younger households and households with higher incomes require more calls to complete an interview due to telephone-screening devices. These households also tend to have higher refusal rates (Zmud, 2003). Evidence suggests that non-contacts lead active lifestyles and are highly mobile. In terms of travel surveys, absence of data from these households results in an under-estimation of trip rates. In addition, potential refusers possess different demographic characteristics than non-contacts. Higher refusal rates have been found among the elderly and low-educated persons (Kurth et al., 2001). As part of this project, research was undertaken to gain some insight into demographic and travel characteristics of non-respondents, why they do not respond, and whether there are any particular elements in survey design and execution that would appeal to non-respondents. Analysis of a call-history file confirmed that households that require fewer call attempts to estab- lish contact and result in a complete response differed, both in terms of mobility and socio-