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Summary of Recommended Standardized Procedures and Guidelines 41 Table 10. Recommended reference trip rates for travel surveys. Trip rate Purpose Mean Value Range All 9.2 8 11 Household HBW 1.7 - HBO 4.7 - NHB 2.8 - Person All 3.4 - quality that is better than average although no clear interpretation of data quality vis-à-vis the non-mobile rate is available at this time. Similarly, person non-mobile rates in excess of 20%, and household non-mobile rates in excess of 1%, indicate below average data quality. Because of the lack of standardization of activity classification and the variety of activity classifi- cation schemes used in transportation at this stage, it is not recommended that activity rates be used to measure data quality. If future travel surveys adopt consistent definitions of activities, as proposed elsewhere in this document, activity rates could be reconsidered as an indicator of data quality. Trip rates from numerous studies show reasonable stability among studies. As expected, trip rates at the person level demonstrate less variability than trip rates at the household level due to the influence of household size. However, household trip rates are frequently quoted and have formed the basis of validation checks in the past. Therefore, it is recommended that the trip rates in Table 10, which include household trip rates, serve as reference values for future travel sur- veys. Deviations from these values must be interpreted by the analyst because the specific rela- tionship between trip rates and data quality has not been established. Note that the trip rates shown in Table 10 are linked, unweighted, person trips per day. 2.7.3 Q-3: Coverage Error Coverage error in surveys is the error incurred by having a sampling frame that deviates from the survey population. It is usually considered to represent the failure to include all the units of the target population. However, in addition to the "under-coverage" that results from exclusion of valid units in the sampling frame, it is also the unintentional inclusion of units in the survey sample (including duplication of units) that do not belong there. This "over-coverage" can occur, for example, when telephone numbers are used as a sampling frame in an RDD sampling process and, as a consequence, households with multiple telephone lines are sampled at a higher rate than those with a single line. Similarly, "under-coverage" can occur in the same type of sur- vey because some households do not own a telephone or have interrupted telephone service. Coverage error is distinct from non-response error although both result from not obtaining information from units in the survey population. Coverage error results from not having some units in the sampling frame or from having units in the sampling frame that do not belong there. Non-response is failing to obtain a response from units that are within the sampling frame. Fur- ther discussion is to be found in Section 10.3 of the Technical Appendix. It is recommended that 1. Coverage error should be estimated in each future travel survey, using Equation 3: F CE = 1 - x 100 (3) X% where: CE = coverage error in percent; Fx = sample population multiplied by the inverse of the sampling rate; and ~ X = population estimate from an external (reliable) source.