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Internet Access. Is everyone online yet and can we survey them there?. TRB Planning Applications 2009 Session 9A: Travel Survey Methods and Analysis II. Colin Smith & Greg Spitz Resource Systems Group, Inc. Thanks to: Mark Fowler & Tyson Seely of Resource Systems Group Prepared for
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Internet Access Is everyone online yet and can we survey them there? TRB Planning Applications 2009 Session 9A: Travel Survey Methods and Analysis II Colin Smith & Greg Spitz Resource Systems Group, Inc. Thanks to: Mark Fowler & Tyson Seely of Resource Systems Group Prepared for TRB Planning Applications Conference May 18, 2009 Greg Spitz Director of Transportation Market Research RSG, Inc
Research Question For transportation surveys: Can we obtain a valid sample if we only recruit and survey respondents who have internet access? What are the populations we are interested in sampling? • Toll road customers • Transit riders • Potential users of a new or changed road or transit system • Geographical areas of travelers
Research Question • What do we mean by “recruit”? • Contacting potential respondents by mail, directly on facilities, at activity locations, through customer lists, or through paid sample providers • Why would we only want to recruit respondents who have the ability to respond online? • Online surveying is generally less expensive than other methods and can be used to administer very complex survey instruments
Research Question What is the problem we are concerned about? • Coverage error • Dillman describes coverage error as, “the result of not allowing all members of the survey population to have an equal or known chance of being sampled for participation in the survey” • Do we introduce coverage error if we only recruit respondents with internet access?
Coverage Error: Telephone Surveys • Rapid growth in the prevalence of cell phones raises concerns about undercoverage in RDD telephone samples (Tucker et al, 2007) • Wireless-only households made up 15% of US households in 2007, significantly higher in 2009 • Undercoverage is clearest for low income young adults: proportion of wireless-only households was 32% in 2006.
Coverage Error: Telephone Surveys • Rate of increase in wireless-only households is rising rapidly in the recession • Additional15% of households have both landlines and cell phones but take few or no calls on their landlines (e.g., fax/internet line) • Attempts are being made to supplement RDD samples with cell phone samples—but cell phones are not geographically based and are more costly and produces relatively low rates of participation (Link et al, 2007)
Coverage error: Internet Surveys Source: Pew Internet and American Life Project
Coverage error: Internet Surveys Those over 65, with low income and with lower levels of educational attainment have lower rates of internet access. Source: Pew Internet and American Life Project
Case Study 1: Toll Road Market Research Computer-based stated preference surveys designed to estimate the values of travel time savings of potential users of road-pricing projects
Project Locations Anchorage, AK San Francisco, CA Denver, CO Atlanta, GA Jackson, MS Orlando, FL Broward County, FL
Recruitment Approach Recruitment was carried out in several ways for these surveys; the samples can be grouped into four categories: The analysis presented today is based on the data collected at activity locations (e.g. shopping malls) where respondents are intercepted at random
Case Study 1: Conclusions • Toll road studies have higher proportion of internet access • This is likely due to income effects of vehicle ownership • Internet access sample is higher income and owns more vehicles • The sample with internet access is not significantly different along income, age, and vehicle ownership dimensions than the full sample • Collecting data only from an internet access sample would not lead to significant coverage error, but does undercover a small and different part of the population
Case Study 2: Transit On Board OD Studies System wide OD study of all CTA’s bus and subway lines Chicago, IL New York City, NY OD study of Metro North Railroad’s three east-of-Hudson commuter rail lines
Recruitment Approach • Transit origin-destination surveys • Large customer sample • Travel patterns obtained • Onboard recruitment • Large samples: 92,000 responses in New York and 34,000 in Chicago • New York: all inbound Metro North trains surveyed • Chicago: a sample of runs of every CTA bus route and subway line
Coverage Error: Phone Respondents vs. Email Respondents • Telephone respondents are not landline-only • Research question: is the email sample representative of the overall sample, or would surveying by email lead to coverage error?
Coverage Error: Phone Sample v Email Sample Overall sample Phone sample Phone only Not available for future research Email and phone Email sample Email only
Case Study 2: Conclusions • Large samples mean high confidence that overall sample is representative • Very different demographic profiles across the 2 studies • In both studies, email and phone samples are representative of the overall sample • Collecting an email or phone contact list for future market research from surveyed transit riders does not lead to significant coverage error
Research Conclusion Do we introduce coverage error if we only recruit respondents with internet access? Two cases: Potential road users intercepted at activity locations such as shopping malls Transit riders surveyed on board the transit vehicle In both cases, the sample of survey respondents with internet was very close to the overall sample, though weighting of income and age should be conducted Therefore, we would not introduce significant coverage error if we only surveyed those with internet access, and this can be mitigated through weighting to population proportion.
Final Thoughts • No survey method completely eliminates coverage error • Anecdotally, web is perceived to have the highest coverage error in our industry • Reality is that the web is as good or better than commonly used RDD and other survey methods to represent transportation study populations • Broad based geographic studies are the most difficult to sample. The best way to sample such studies is to use addressed-based sample and not RDD or purchased sample • Many broad based studies still conducted using RDD and this creates serious coverage error issues
