We did not see a similar increase when the federal

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We did not see a similar increase when the federal maximum speed limit was repealed, though not all the states changed their speed limits at this time. A follow-up study would examine whether the fatality rates differed between states that did and did not increase speed limits (see Example 3.2).
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Chance/Rossman, 2015 ISCAM III Investigation A 10 Discussion: Statistical analysis often involves looking for patterns in data. In the age of the “data deluge,” this is becoming increasingly important. This investigation should have raised your awareness of several key issues that will permeate the studies you analyze in this text. x It’s important to determine which variables are most relevant to the research question and whether you can collect the data you need to answer the question x Simple graphs can be very informative, but you should also take care in considering the most meaningful variable representation of what you are studying even before you begin graphing. x It is often misleading to view results in isolation. The change in traffic fatalities needs to be considered in the context of other years and other changes. x It is imperative to consider variability. The percentage change will fluctuate from year to year naturally, and we should not make large-scale conclusions of the impact of policy changes without considering the size of a change with respect to the natural variability. Standardizing is one way to interpret an observation’s relative position in a distribution in terms of “how many standard deviations is the observation from the mean = (observation mean) / standard deviation. x It is very difficult to isolate the cause of a change. There easily could have been other reasons for a decrease in traffic deaths between two years even just awareness of the issue could have changed drivers’ behavior. A key skill in statistical analysis is communicating the results of your analysis to different audiences, including those with little experience with quantitative data. With each investigation we will present a Study Conclusions box as above as a model for an effective summary of the analysis of the research question, which often includes statements of new questions. With most investigations we will also provide a follow-up practice problem for you to try on your own to assess your understanding of the material. Practice Problem A Go to http://www-fars.nhtsa.dot.gov/ and x Select the FARS Data Tables tab (it should be orange). x Press the Trends tab (so it turns orange) underneath. x In the next line of tabs, select General. x Use the pull-down menu to select a state other than California. x Scroll down to find the Fatalities and Fatality Rates table. Then click on the Export to export the data to a text or Excel file.
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