Could someone please link the original paper by Dr. Nigel Perry here. Simply seeing Dr. Perry's graph without the context of the paper doesn't tell us much. In the paper here:
http://www.chapmancentral.co.uk/wiki..._helmet_debate
tries to show a table that goes from 1990 to 1996 to this graph, which goes from 1988 to 1996.
Again, it is getting busy trombone, and I will answer your question logic. This post took only a couple of minutes, while researching the logic for your post will take longer, as you have thought it through. And...classes started today, a week late as the hurricane evacuated New Orleans.
John