You Conduct a Quasi-Experiment to Assess the Impact of Raising

You conduct a quasi-experiment to assess the impact of raising the speed limit from 55 to 65 miles per hour. You find out that there were more accidents at higher speeds. Although it is tempting to say that raising the speed limit caused higher accident rates, you must be careful because:

A) you did not actually manipulate an independent variable.
B) drivers may have exceeded the 65-mile-per-hour speed limit.
C) other variables (for example, cheaper gasoline or the season of the year during which the change was instituted) may also be affecting accident rates.
D) None of the answers is correct.