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Summary of Recommended Standardized Procedures and Guidelines 35 Table 8. (Continued). Primary Code Secondary Categories Code Tertiary Categories Code Category Pick up or drop off other 083 Pick up someone or get picked up 0831 people/get picked up or Drop off someone or get dropped off 0832 dropped off (private car, car/van pool, shuttle/limousine) Activities related to bus, 084 Wait for/get on vehicle 0841 public transit, and group Leave/get off vehicle 0842 rides (except car/van pool and shuttle/limousine) Change travel mode 085 n.f.d. 0850 Other specified activity 089 n.f.d. 0890 related to accompanying others or travel-related No activity 09 No activity 091 n.f.d. 0910 No recorded activity 092 n.f.d. 0920 No further activity recorded 093 n.f.d. 0930 Other 99 n.f.d. 990 n.f.d. 9900 Table 3 used in Section 2.1.2 also provides categories for a number of questions which are con- sistent with the coding procedure proposed here. It is recommended that the codes in that table also be adopted as a consistent set of codes for the variables listed therein. 2.6 Data Analysis and Expansion 2.6.1 A-1: Assessing Sample Bias Sample bias is a systematic error in sample survey data. It reflects a consistent deviation of sample values from true values in the population. Bias can occur within individual observations when, for example, a faulty measurement device is used and a consistent error is introduced into each observation. Of course, bias in individual observations is carried through to aggregate val- ues of the sample such as means and proportions. However, even if individual observations are not biased, if the sample is not representative of the population, assumptions that it is produces biased estimates of the population. This is a condition that can occur quite readily because draw- ing a truly random sample from the population is complicated by factors such as the practical difficulty of establishing a perfect sampling frame, having an equal likelihood of contacting each sampling unit, and obtaining full response from each sampling unit. The issues of sample bias and its assessment are discussed in detail in Section 9.1 of the Technical Appendix. It is recommended that the following standardized procedures be adopted with respect to bias in travel surveys: 1. Each travel survey should test for bias. 2. The following variables should be used to test for bias: Household size; Vehicle availability; Household income (if collected); Race of each person in the household; Age of each person in the household; and Gender of each person in the household. 3. The variables should be measured as follows: Household size: mean value; Vehicle availability: categories of 0, 1, 2, and 3+;