Photo: Ekaterina Pokrovsky/Shutterstock
In the summer of 2023, 28 U.S. states, including Alabama, started banning handheld cell phones while driving. This news story from the Birmingham News (AL) shows data from an interrupted time series design that tracked people's distracted phone use through automatically collected phone movement data. They plot out the results of four states (OH, AL, MI, and MO) and report values for 3 months before these hands-free laws began and for 10 months afterwards. Take a look at the figure here: https://tinyurl.com/23tt7xmq
a) Why might this design be called an "interrupted time series"? (Chapter 13 will have definitions to help you.)
b) Even though there are four states depicted on the main figure, this is probably not a "non-equivalent control group interrupted time series design." That's because there are no examples of states that did not implement a hands-free ban that summer. When we are trying to decide whether the hands-free cell phone ban reduced distracted driving, it would be helpful to have a set of states with no ban. Why? (You might use the term "history threat" in your answer).
c) Based on the figure, what do you think of the effect size in each state?
d) This study's interrupted time series has only about three months of data from before the time when the bans went into effect. Why might we want more than that as a baseline?
You might wish to look through an official report with bar-graph versions of the data, which can be viewed here: https://m.cmtelematics.com/hubfs/A%20Road%20Map%20for%20Safer%20Roads.pdf
I'm grateful to Stephen Chew, who shared this story with me.
