Here's a study that took advantage of "4-20", an unofficial holiday which people celebrate by holding pot-smoking parties starting at 4:20pm. Here's how the quasi-experiment was described in a New York Times story:
Researchers used 25 years of data on car crashes in the United States in which at least one person died. They compared the number of fatal accidents between 4:20 p.m. and midnight on April 20 each year with accidents during the same hours one week before and one week after that date.
a) What are the "independent" and dependent variables in this study? (And why did I put independent variable in quotes?)
Here's how the journalist described the results:
Before 4:20 p.m. there was no difference between the number of fatalities on April 20 and the number on the nearby dates. But from 4:20 p.m. to midnight, there was a 12 percent increased risk of a fatal car crash on April 20 compared with the control dates.
b) Of the four quasi experimental designs, which seems to be the best fit: Non-equivalent control group posttest only? Non-equivalent control group pretest-posttest? Interrupted time series design? Non-equivalent control group posttest-only design?
c) Sketch a graph of the results described.
d) The Times reported that "The increased risk was particularly large in drivers 20 and younger." Why might the researchers have included this detail?
e) The Times's headline read, "Marijuana Use Tied to Fatal Car Crashes". What kind of claim is this? (Frequency, Association, or Cause?)
f) To what extent can these results support a causal claim about marijuana causing crashes? Apply the three causal criteria to this design and results.