Wordwizard

Can one say

a. Fifty percent of homeless men have suffered brain damage. Not fifty percent of the male population at large have suffered brain damage.
b. Fifty percent of homeless men have suffered brain damage. But not fifty percent of the male population at large have suffered brain damage.

c. Fifty percent of homeless men have suffered brain damage. Not fifty percent of men at large, but only X percent, have suffered brain damage.
d. Fifty percent of homeless men have suffered brain damage. But not fifty percent of men at large, but only X percent, have suffered brain damage.
?

The fifty percent figure seems more or less correct. I could not find the percentage of men who had brain damage in the general public and used X instead of a figure.

(a) and (b) sound incorrect to me.

I would say one could use the 'structure' in (a) and (b) when one wants to emphasis the fact that a figure is small.
e. 'Not two percent of the population know that Einstein published his article on special relativity in 1905.'

(e) seems fine to me since it emphasizes the fact that very few people 'know that'.

Am I correct?

Many thanks.

I am afraid I don't know what you are trying to say about brain damage and homelessness.
.
As for Einstein, "Fewer than two percent of the population know that Einstein published his article on special relativity in 1905".

Bob I think you are right. I have read these a few times and there is no real sense of what they are trying to say.

The point of "not 50%" is not a phrase that would have much use but your addition of "fewer than" creates a range that is then useful, as would "greater than".

PS one of the rare (in my opinion) opportunities might be: cars are more dangerous when travelling at velocities of not nought miles per hour.