Hence, the total sample size of 6,000 drivers in the survey is comprised of four independent samples of approximately 1,500 respondents, each. The survey was conducted between February 20 and April 11, 1997. The telephone interviews averaged 30 minutes in length. A total of 6,000 interviews were completed with a participation rate of 73.5 percent. The completed interviews were weighted to correct for selection bias as a result of the number of telephone lines and eligible respondents in the household.
All sample surveys are subject to sampling variability or sampling error.
The sampling error is the range within which sample estimates are expected to vary from true population values. Each table is labeled to show the appropriate, unweighted base.