In Western Australia, handheld cell phone use while driving has been banned since 2001, but hands-free devices are legal. A study (published in the British Medical Journal in 2005) of 456 drivers in Perth who had been in a crash that put them in a hospital emergency room observed if they were using a cell phone before the crash and if they were using a cell phone during an earlier period when no accident occurred. Thus, each driver served as his or her own control group in the study.
a. In comparing rates of cell phone use for those in accidents and those not in accidents, should we use methods for independent samples or for dependent samples? Explain.
b. The study found that using a hands-free cell phone quadruples the risk of getting into a crash with serious injuries. Identify the statistic used to compare the groups, and interpret it.