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Procedures and Measures for Further Research 63 selecting a set of key variables from among the core variables required in any survey. An alterna- tive approach could be to identify those variables most relevant to the purpose, or purposes, of the survey and to measure the sampling error on each variable (TMIP, 1996a). Regardless of the approach taken, the determination of the key variables should be related back also to the minimum specification of questions already developed as part of this project (see Section 2.1.1). To illustrate the effects of the standardized procedure and its interpretation, it is recommend that sampling errors be calculated for two or three recent surveys on the key variables specified. 4.3 Other Research Directions In this section, we outline briefly ideas that surfaced during the execution of this research. A number of these have been partially researched in this project, but further work is seen as being warranted to complete what has been started and to develop standardized procedures or consis- tent guidelines. 4.3.1 Cell Phones Cell phone usage has grown at a phenomenal rate over the past decade and has profound impli- cations for the way in which surveys are conducted. In 2003, cell phones composed about 43% of all U.S. phones, which represented an increase of 37% since the year 2000 (USA Today, 2003). In addition to this, many households are now moving away from landline phones and using cell phones exclusively. In June 2003, the Federal Communications Commission reported that in the period since the year 2000, landline phones decreased by more than 5 million, or around 3% (USA Today, 2003). The majority of cell phones are unlisted, which means that it will become increas- ingly difficult to contact large sections of the population through RDD. In our opinion, research on the use of cell phones should be focused on two key areas. First, there is a need to determine the effects that growing cell phone use will have on household travel surveys. Specifically, more needs to be known about members of the population who are moving toward exclusive use of cell phones. It is likely that such information could be obtained from a federal government agency such as the Federal Communications Commission or a communica- tions industry group such as the Cellular Telecommunications and Internet Association. Second, once it is known which segments of the population will become increasingly difficult to contact, alternative strategies will need to be developed to find new ways of reaching such groups. Although the increased take up of cell phones may create difficulties in reaching certain sections of the population, it is worth noting they may also create some new opportunities. Once initial contact has been made and a person has agreed to take part in a survey, they may actually be easier to contact (e.g., for recall interviews) than would have previously been the case. If a per- sonalized interview technique were adopted, such as the Brög method, a relationship could even be established whereby the interviewer would deal with their contacts as "clients," who would be free to call their interviewer or "agent" whenever they felt it necessary. It is recommended that these kinds of opportunities be explored as part of any research conducted on the impacts of increasing cell phone use among the population. One of the problems with using cell phones is that calls received incur the same cost as a call placed and, therefore, the use of cell phones in a survey would impose a cost on survey respon- dents, which is generally considered a violation of ethical standards for surveys. In the past, it has been relatively easy to exclude cell phones because certain blocks of numbers were reserved by telephone companies for allocation to cell phones. However, this is eroding as number porta- bility allows people to shift their landline number to a cell phone. One possibility is that this trend will further damage the potential of using the telephone as a means to recruit and retrieve