A study tested long-term IQ changes of people who smoked marijuana as teenagers. The study is described here.
...participants were tested for IQ twice in the study: once at age 13, and once at age 38. ...researchers found that those who used marijuana regularly before the age of 18 showed a drop in IQ score. The same drop in IQ wasn't found for those who began to smoke up regularly after 18.
b) Make a graph of the results described in this sentence from the journalist's report:
Essentially, the more participants smoked before age 18, the heavier the IQ loss recorded by the researchers.
c) Finally, the journalist reports that:
...while the study's findings seem to give some weight to the "stoner" stereotype of longtime heavy cannabis users, other factors, such as depression, could also explain both marijuana use and a lower IQ in participants.
What kind of validity is this sentence addressing? What kind of study would be able to rule out the idea that "other factors, such as depression" are responsible for the lower IQ and marijuana use? (There a couple of study designs that could address this issue.)