[Ferguson:] This problem isn’t going away. Ask the French. About 8% of the French population is Muslim, which is roughly the proportion the Pew Research Centre projects it will be in Britain by 2030. The French authorities estimate that they have 11,400 radical Islamists. And about 60% to 70% of the French prison population is Muslim. If you haven’t read Michel Houellebecq’s Submission, about an Islamist takeover of France, now might be a good time. Alternatively, you can “drink tea and jolly well carry on” — though it’s hard to do that when your head’s in the sand.
This [60% to 70%] figure is from a sociologist's survey of four prisons in northern Paris, it is not nationally representative.
[Ferguson:] Sorry, you can't call Khosrokhavar's % a "falsehood" if all you're offering is a lower estimate. (And 27% looks too low.)
Khosrokhavar's figures are for four prisons. He doesn't say this is nationally representative. You do. That's the falsehood.
I emailed him, and he says his best guess is that the national figure is about half what you say it is.