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58 Standardized Procedures for Personal Travel Surveys reason for this is that unless the finite population correction factor is large, which will rarely be the case in urban area surveys, the error levels of a sample will not be dependent on the regional population. The specifics of the sample size will be dependent, however, on the use to which the data will be put and the sample design--that is, stratification, clustering, or other sampling method. 4.2.11 S-2: Sizes of and Procedures for Surveying Augment Samples Household travel surveys often require augmentation because of a lack of rare behaviors in the collected data and the problems of sampling to include them. Rare behaviors in the United States and other countries include transit riding and bicycling, among others. In most metropolitan areas in the United States, the proportion of transit riders varies between about 0.5% and 5% of all trips. These low percentages may mean that small samples will contain very few transit trips for generalization to the entire population and are certainly too small for modeling mode choice. Research is needed to determine when an augment sample is necessary. A review of various regional statistics and also past reported problems with insufficient observations on specific aspects of a household travel survey would help identify the types of situations where an augment sample would be required and how such data could be used. There is also a need for guidance on the size of the augment samples. Because augment samples are generally collected for modeling purposes, there is usually a focus on collecting data on specific rare mode choices for estimating mode-choice models. In light of this, guide- lines would probably need to be based on sample sizes required for reliable estimation of cur- rent mode-choice models. It would be important to consider that the sample needed must support segmentation by trip purpose, at least into home-based work, home-based non- work, and non-home-based. It is therefore suggested that research examine the split of pur- poses within such trips as transit, bicycle, and walk and develop recommendations on sam- ple sizes from this. For example, it has been suggested in the past that at least 300 observations are needed on each mode to obtain reasonable estimates of mode-choice model parameters in a logit model. Assume that models of the three trip purposes mentioned above are to be estimated and that approxi- mately 60% of transit trips are home-based work, 25% are home-based non-work, and 15% are non-home-based. In this case, the need is to have 300 samples in the non-home-based category for that model, which would generate the requirement that 2,000 transit trips are measured in total. If we were to find that the average rate of transit trip making by transit-riding households is 4 transit trips per day, then this would translate to the requirement for a total sample of 500 transit- riding households. If it is further assumed that the general household sample will produce about 50 transit-riding households, then the augment sample would need to be 450 transit- riding households. This provides an example of how the guidance would be developed for aug- ment samples. 4.2.12 S-3: Collecting Augment Samples In addition to the sample sizes and procedures for surveying augment samples, there is also a need to examine how data should be collected on the augment sample. For example, a number of past household surveys have used an on-board bus survey to augment the sample for transit trips. However, the nature of the on-board survey is usually significantly different from the nature of the household travel survey. In the event that such a mechanism is to be used, there are certain requirements that need to be spelled out for the on-board transit survey. Similar issues would apply if special surveys are conducted with other subgroups of the population on a choice-based or other sampling basis.