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20 Standardized Procedures for Personal Travel Surveys 5. Conduct refusal conversion. Refusal conversion should be conducted, with a maximum of five attempts to convert initial soft refusals. This recommendation is also stated for initially non-contactable households and is shown in Section 2.2.1 of this report. 2.2.5 D-7: Item Non-Response Item non-response has been defined as either the failure to obtain an answer to a specific ques- tion or the failure to obtain a true and complete answer. Thus, item non-response occurs not only as a result of data being missing but also when incorrect data are provided. Invalid data are data items whose values are beyond the possible or feasible range of that item. Inconsistent data are data items whose values are inconsistent with the values of other data items of the respondent. Item non-response is closely linked to several other items discussed in this report. First, it is linked to the definition of a complete household addressed in Section 2.2.3 because it is only when item non-response is within tolerable limits that a responding household is considered complete. Second, it relates to survey design and survey execution because the form in which the questions are posed and the manner in which the survey is conducted are known to have a sig- nificant impact on item non-response. A more detailed discussion of this issue is to be found in Section 5.5 of the Technical Appendix. The overall approach should be that item non-response should be minimized by good survey design and good survey execution. To achieve this, the following standardized procedures are recommended: 1. For CATI, computer-assisted personal interviewing (CAPI), and Internet surveys, admin- istration of the survey should be programmed to require that a response is obtained on each item. 2. Mail-back surveys should be edited immediately upon receipt so that respondents can be re-contacted to query missing or incorrect data items while the survey is still fresh in their memory. 3. Item non-response should be considered to include items where values are missing, where the respondent has indicated that they "don't know," and where the respondent has refused to answer. 4. An overall estimate of item non-response should be obtained from the level of non- response on each of the following items: · Travel mode; · Driver license status; · Start time and end time of trip or travel time of trip (if only travel time of trip is reported); and · Vehicle occupancy. A statistic, which is the average item non-response among the above items, should be used as the overall measure of non-response in the data and should be expressed as a percentage. 2.2.6 D-8: Unit Non-Response There are two broad categories of unit non-response: refusals (hard refusals, soft refusals, and ter- minations) and non-contacts (for CATI surveys, these are busy, no reply, and answering machines). High rates of unit non-response are generally associated with non-response error. Non-response error is a function of the non-response rate and the difference between respondents and non- respondents on any statistic of interest. A lower unit non-response rate is desired because this reduces the incidence of non-response bias. This issue is discussed at length in Section 5.5 of the Technical Appendix, together with the results of a non-response survey conducted as part of this research.