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Procedures and Measures for Further Research 61 assessing the quality of any given survey by comparing the variance measured in a specific survey to the default value for each attribute of interest. Variances either much smaller or much larger might indicate potential problems in the survey. It is recommended that research on this issue be undertaken in conjunction with work on stratification options (see Section 4.2.13) and the specification of sampling error requirements (see Section 4.2.14). It is suggested that variances be estimated for a variety of relevant variables and from as many different data sets as possible. These could include trip rates by purpose and overall per person and per household, mode shares by purpose, and average trip lengths by pur- pose. Recommendations should suggest a mean or median variance that could be used as a default for sample design for each appropriate variable. The implications of using default vari- ances for setting sample sizes would need to be checked by comparing them with the results of using actual variances for several recent surveys. In the absence of any local information, these variances could be used to estimate stratification, sampling rates, and sampling errors. Perhaps of even greater use would be to determine default values of coefficients of variation (cv) that could be used in determining sample size because sample size and error computation also require knowledge of the mean. 4.2.16 P-1: Focus Groups The transportation profession has only recently begun to understand and appreciate the poten- tial of focus groups. These have been a mainstay of the marketing profession for quite some time and have enormous applicability to various aspects of survey design. While some personal travel surveys are conducted by marketing firms that may be familiar with focus groups, many surveys are conducted by transportation engineering and planning firms who are not familiar with them. Only a small minority of transportation surveys has used focus groups to help with the design of the survey; yet, this is a powerful mechanism to improve the design and quality of a survey. In the design process, one or more focus groups can provide important information in an effective man- ner and may be much more cost-effective than a number of pretests. While extremely useful, focus groups can probably be considered as good or better practice rather than basic practice in trans- portation surveys. Guidelines or a primer on how focus groups could be set up and used in household and per- sonal travel surveys would appear to be very useful. Among the issues that need to be addressed are the following: How many focus groups are needed? What is the optimum size of a focus group? How should focus groups be used to test a travel survey? How can a focus group be recruited? How much is it necessary or desirable to pay focus group members to participate? Should focus group members receive survey materials prior to meeting? How should a meeting location and time be arranged? What qualifications are needed to facilitate a focus group? Should focus group discussions be recorded? What benefits arise from using focus groups? How is a focus group conducted? Literature from marketing and other areas should be consulted to prepare responses to these and other important questions and could help determine the extent to which focus groups are subject to standardized procedures. If possible, it may be useful to field test a small focus group to provide additional information for any proposed standards or guidelines.