Of course one issue with this analysis is that after

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Of course, one issue with this analysis is that after the repeal of the federal speed limit different states chose to implement changes to the speed limits in different years, so focusing on the 1994-95 change is too simplistic. One way to account for this source of variability is by focusing on an individual state. The CAFatalities.xls file contains the number of fatalities and population size (in thousands) for 1993- 2012 from www-fars.nhtsa.dot.gov/Trends/TrendsGeneral.aspx. (n) What was (roughly) the California population in 1993? (o) Use Excel to compute the fatality rates per thousand residents (e.g., In cell E2, enter =C2/D2 , then put the cursor back over that cell and double click on the lower right corner to “fill down” the column. Remember to name the column!). What was the fa tality rate in 1993? (Include “measurement units” with your answer.) (p) Now compute the percentage change in fatality rates (e.g., In cell F3, enter =(E3-E2)/E2*100 and fill down). What was the percentage change from 1993 to 1994? Was it positive or negative?
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Chance/Rossman, 2015 ISCAM III Investigation A 9 (q) Use technology to create a dotplot or histogram of the percentage change in fatality rates for California during this time frame, and calculate the mean and standard deviation. Compare these results to what you found above for the entire United States. (r) In particular, is the standard deviation in (q) larger or smaller than for the U.S. data? Give some explanations why the standard deviation has changed this way. [ Hint : Focus on issues other than the sample size (the number of years of data), because the number of observations is not generally an indicator of how large the “typical deviation from the mean” will be. In fact, if we consider the smalle r population size in CA, we might expect there to be more variability in the rate of accidents per year…] (s) On Jan. 7, 1996, California allowed rural speed limits to increase from 65 to 70 mph and urban roads to increase from 55 to 65 mph. What is the percentage change in the fatality rate for California between 1995 and 1996? Is this a particularly large value in the distribution for California? [Be sure to justify your answer.] (t) Suppose we did find a large increase or decrease in a state’ s fatality rate before and after the change in speed limit laws. Would it be reasonable to conclude that the change in speed limit laws caused the change in fatality rate? Why or why not? What else should we consider? Study Conclusions Many researchers are interested in whether changes in speed limits have had an impact on the number of traffic fatalities. In this investigation, we focused on the number of fatalities per 100,000 in the United States. (You could also look at other rates like fatalities per 100 million miles traveled.) We see a decreasing trend in the fatality rate over time, though the rates appear to be leveling off after about 1950. The second largest decrease (more than one standard deviation from the mean) occurred in 1976, after the imposition of a federal maximum speed limit.
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