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24 Standardized Procedures for Personal Travel Surveys instrument, sampling, etc. During the design phase, several sequential pretests could be conducted to test various refinements of the instrument, protocol, sampling, etc. Second, it is recommended that one or more pretests and/or one or more pilot surveys should be an essential step in all transportation surveys unless there are specific circumstances that ren- der such a step unnecessary and unlikely to produce useful information. It is further recommended that the following guidelines with respect to pilot tests and pretests be adopted: 1. In any survey in which interviewers will interact with respondents, the pilot survey or pretest should include listening in to interviewers to determine how they interact with potential respondents, how well they keep to the script of the survey, and whether the script causes difficulties in conversational style. 2. In any survey that uses interviewers or observers, there should be a debriefing with those used in the pilot survey or pretest to determine whether difficulties were experienced in handling survey procedures, questionnaires or other materials, scripts, etc. 3. If it has been 10 years or more since the last time a survey was done, a pilot survey should always be undertaken because the changes in population that will have occurred will ren- der any past experience irrelevant. 2.3.2 P-3: Sample Sizes for Pretests and Pilot Surveys Because we recommend that pretests and/or pilot surveys be conducted in all future travel sur- veys, it is appropriate to establish the required sample size of these initial tests or surveys. For further discussion, the reader is referred to Section 6.2 of the Technical Appendix. It is recommended that the following standardized procedures be adopted by the profession: 1. Whenever possible, the main sample should be drawn first and the pilot survey or pretest sam- ple drawn only from those households or persons who were not drawn for the main sample. When the pilot survey or pretest is being conducted to determine the sample size required for the main survey, two options are possible. The first option is that a main sample can be drawn that is expected to be more than sufficient in size. The pilot survey or pretest sam- ple can then still be drawn subsequently from those households or persons who will not be included in the main sample under any likely circumstances. The second option is to draw the pilot survey or pretest sample at random from the total population and then be sure to exclude all such drawings from the population for drawing the main sample. The former of these two is the preferred method. 2. No pretest or pilot survey should use a sample of less than 30 completed households or respon- dents. Exercises using smaller samples than this should be regarded as preliminary tests and pre-pilot surveys and should always be followed by a pretest or pilot survey with at least a 30 respondent sample size. 3. The minimum sample sizes shown in Table 6 should be used in all pilot surveys and appropri- ate pretests. 2.4 Survey Implementation 2.4.1 E-2: Ethics Ethics describe minimum acceptable standards of conduct or practice. In travel surveys, this relates to how a survey agency conducts itself with respect to those interviewed, the client, any subcontractors, and the public as a whole. It also relates to a survey agency's actions following the data collection process when data are cleaned, coded, analyzed, and archived.
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Summary of Recommended Standardized Procedures and Guidelines 25 Table 6. Sample sizes required for specified levels of accuracy. Measure Assumed Desired Sample Measure Assumed Desired Assumed Sample Value Accuracy Size Value Accuracy Variance Size Response Rate 50% ±5% 384 Household or 10 ±1 100 384 50% ±10% 96 Person Trip Rate 10 ±2 100 96 50% ±15% 43 10 ±3 100 43 50% ±20% 24 10 ±4 100 24 60% or 40% ±5% 369 10 ±1 50 192 60% or 40% ±10% 92 10 ±2 50 48 60% or 40% ±15% 41 10 ±3 50 21 60% or 40% ±20% 23 10 ±4 50 12 75% or 25% ±5% 288 7 ±0.5 70 1076 75% or 25% ±10% 72 7 ±1 70 269 75% or 25% ±15% 32 7 ±1.5 70 120 75% or 25% ±20% 18 7 ±2 70 67 Non-response 10% ±3% 384 7 ±0.5 50 768 to a Question 10% ±5% 138 7 ±1 50 192 10% ±8% 54 7 ±1.5 50 85 10% ±10% 35 7 ±2 50 48 20% ±3% 683 4 ±0.4 40 960 20% ±5% 246 4 ±0.8 40 240 20% ±8% 96 4 ±1 40 154 20% ±10% 61 4 ±1.5 40 68 30% ±3% 896 4 ±0.4 16 384 30% ±5% 323 4 ±0.8 16 96 30% ±8% 126 4 ±1 16 61 30% ±10% 81 4 ±1.5 16 27 Ethics reflect what all stakeholders may consider "fair" or "reasonable" conduct by those involved. In practical terms, the application of ethics involves implementation of precautions to protect those affected from adverse effects. Ethics protect the rights of individuals and groups and serve to reduce public disapproval and criticism of what is done. A review of the ethics standards used in various other areas of surveying is provided in Sections 2.2.2 and 7.1 of the Technical Appendix. After reviewing documents prepared by various associations on different aspects of ethical conduct in the execution of travel surveys, it is recommended that the following ethical conduct be observed in all future travel surveys: 1. The anonymity of the persons surveyed, and the confidentiality of the information they provide, must be protected at all times; 2. A survey respondent may not be sold anything or asked for money as part of the survey; 3. Persons must be contacted at reasonable times to participate in the survey and must be allowed to reschedule participation in the survey to a different time if that is more con- venient for them; 4. Survey personnel must be prepared to divulge their own name, the identity of the research company they represent, the identity of the agency that commissioned the study, and the nature of the survey being conducted, if requested by a respondent; 5. Children under the age of 15 may not be interviewed without the consent of a parent or responsible adult; 6. A respondent's decision to refuse participation in a survey, not answer specific questions in the survey, or terminate an interview while in progress must be respected if that is the respondents' firm decision; 7. Respondents may not be surveyed or observed without their knowledge: methods of data collection such as the use of hidden tape recorders, cameras, one-way mirrors, or invisible identifiers on mail questionnaires may only be used in a survey if the method has been fully disclosed to the respondent and the respondent agrees to its use.