Researchers in New Zealand interviewed 907 drivers at age 21. They had data on traffic accidents and they asked the drivers about marijuana use. Here are data on the numbers of accidents caused by these drivers at age 19, broken down by marijuana use at the same age:

a. Make a graph that displays the accident rate for each category of marijuana use. Is there evidence of an association between marijuana use and traffic accidents? Justify your answer.

b. Explain why we can’t conclude that marijuana use causes accidents based on this study.

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Marijuana use per year Never 1210 times 11250 times 511 times Number of drivers 452 229 70 156 Accidents caused 59 36 15 50