Rather simply. I'd do a long-term study of 100 or so randomly selected children who were readers (defined as 20 or more books per year), compile a list of books they read, and then analyze said list by amount, genre, etc. Start with them at age 12, and end at age 22. Do a followup 5 and 10 years after graduation to see what their reading habits were, and what career paths they followed. Compare it to a demographically similar group as well as the general population.Actually the study concluded or expired in July so it should be or is already available.
The second is an essay with anecdotes. The third was a report which stated that there were women who were inspired to thier career by scienfiction.
Realize that a study of this nature necessarily has to build its statistics on anecdotes, or specifically, personal anecdotes. When a scientists or engineer goes on record to say that he was influence by science fiction in the direction of his choice of career, that is a data point in such a study. How else would you gather that kind of data?
Studies like this are immensely powerful in determining useful long-term trend data and educational usefulness. They are reasonably hard to perform however.
Less rigorous, but also scientifically based studies could be devised.
I do not find that it is ever necessary to rely on anecdotes.There are scientists and engineers who have gone on record to say so.
Konstantin Tsiolkovsky, Herman Oberth, Robert Goddard, and Werner Von Braun have stated in biographies and interviews to have been inspired by Jules Verne.
http://www.meaus.com/spacePioneer.html
http://inventors.about.com/od/bstartinventors/a/Von_Braun.htm
http://www.amazon.com/Robert-Goddard-Unlocking-Secrets-Science/dp/1584151072
And Scientists have even become science fiction writers. i.e. Isaac Asimov and Author C. Clark.
Well that report in the first link is defintely a study concerning the link between science fiction and directions in careers.
As mentioned before a study of this nature would necessarily rely on personal anecdotes from scientists who would relate thier experiances concerning science fiction and thier choices of careers.