Things They Should Do A Study On: Which religion produces the greatest
proportion of atheists?

Study children who are born into each religion and are raised to be actively
religious, and look at how many of them end up being atheist, how many end
up being agnostic, how many end up lapsing, and how many end up converting.

They could also find out which religion causes the most crises in faith, and
in which religion the most people have crises in faith and ultimately decide
to go back to the religion.