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50 Standardized Procedures for Personal Travel Surveys needed relating to these data. These would relate to the size of the task that can and should be presented to respondents (Stopher and Hensher, 2000), as well as issues of how alternative attri- bute levels are set in a stated-choice survey (Stopher, 1998). There is also a need to determine whether attribute levels should be generated in real time or can be pre-set and committed to a printed survey. Consistent instrument designs for the collection of stated-choice data are clearly needed. Many survey firms understand this area relatively poorly, and the whole field of stated-choice research is subject to potential discredit if poor designs are fielded and erroneous conclusions drawn from the data. There are enormous differences of opinion in such areas as · The need for contextual data to be collected at the same time; · The number of possible alternatives that respondents can be asked to handle; · The number of attributes that can be included in the design; · The number of levels of each attribute that can be included; · How far the levels of the attributes can depart from current experience of the respondent; · The number of treatments that an individual respondent can be asked to handle; · Whether the order in which treatments are offered has an effect on choices; · The need for orthogonality in the design; and · How to administer the SP experiment--that is, by paper and pencil, on laptop computer, etc. In addition, there are some survey researchers who do not believe that stated-choice experi- ments are valid and would argue against their use. Research is clearly needed into these various issues. In this case, it appears from the litera- ture that transportation applications of stated-choice surveys are ahead of marketing and other fields that may also use the techniques. As a result of a review of the literature on this topic, the transportation field appears to be addressing issues that other researchers have not con- sidered (Louviere et al., 2000). However, these listed issues have not been researched in trans- portation or elsewhere to date. Hence, to develop standardized procedures for SP data, it will be necessary to undertake research on all of these issues. For the most part, this will require a battery of alternative SP survey designs to test various options in each of the bullets listed above. Several of these can be tested together; the results, in the form of some measure of the quality of the SP sur- vey, can be analyzed through models that seek to explain differences in the quality as a function of the various design variants. At the outset this area was considered to be beyond the scope of this project; it is, therefore, up to future research to establish standards. 4.2 Items Originally Identified and Not Researched 4.2.1 D-2: Who Should Be Surveyed? There is no general consensus about the minimum age for persons included as part of a house- hold travel survey. Traditionally, data have been collected on all household members over the age of 5 years on the assumption that any young children will travel with the non-working mother, who would, therefore, provide complete data on the movements of any very young children. In current society, both parents now work in most households, and it is becoming increasingly dif- ficult to deduce the travel of younger children in the household. In light of this, more household travel surveys are collecting data on all family members, irrespective of age. Another issue that arises in household travel surveys is whether to survey persons living in group quarters. In many instances, those living in group quarters do not travel (e.g., prison inmates, those in hospital, some types of elderly and infirm care facilities); however, other types of group quarters may produce large amounts of travel (e.g., university dormitories and military facilities). Some guidance is