The stereotypical image of a Jew in Middle Ages (and the portrayal of them) was of course "the money-lender". From Ivanhoe's Isaac of York, to Shakespeare's Shylock, to "Проклятый жид, почтенный Соломон" from Pushkin's The Miserly Knight, we see the archetype.
Wikipedia's Shylock page says this on the historical background section:
During Shakespeare's day, money lending was a fairly common occupation among Jews. This was due in part to laws banning Jews from owning farm land or entering guilds, combined with Christians staying out of the profession due to their belief that usury – at that time considered to mean charging interest of any kind on loans – would be a sin.
While the reasoning for why Jews would enter the money lending is quite obvious, what seems to be missing from the picture is all the common boring non-money-lending poor jews (who didn't seem to enter popular culture's picture till Tevye the Dairyman from "Fiddler on the Roof").
So, what interests me is, just HOW common was the money lending occupation among Jews in 1300 to 1700AD period in Europe?
What percentage of Jewish population were money-lenders, vs holding some other profession?
I don't particularly care how you define "money-lender" professionally, but my own personal non-binding suggestion is anyone who either:
Devotes >50% of their work time to this occupation, or
Derives >50% of their income from money-lending.
I don't see any point to distinguishing between money-lenders who lent to gentiles vs. those who lent to other Jews, but if you wish to put that level of finesse into an answer, power to you :)