The Fifth Element or What Elements Are Correlated With CS Success and Does It Matter?
As I indicated in my other question, I have found that there are always a few students who don't seem to get it.  This leads me to suspect that there may well be some small number of persons who are so disadvantaged that it may be effectively impossible for them to become strong programmers.
However here are a few words of cautions:

Even if this is true, what of it? The fact that I have not figured out how to teach Johnny how to code does not necessarily mean Johnny is incapable, and I am still obligated to keep trying to find new ways to reach him.  If, in fact, he really is incapable, I will ultimately fail, but I have no way to distinguish between him and Samantha, who is entirely capable, but has yet to have some key piece of information click into place.

The number of people should be quite small.  For me it has run around 5-10% of students.  I work with a &quot;gifted&quot; population, so that number might be a little smaller than would be found in other populations, but it shouldn't be wildly different.  As you have pointed out, general brightness is a poor indicator.
If you're finding that &gt;25% of your students are seriously struggling, then there are additional factors beyond &quot;student ability&quot; that you need to be examining.