You might be interested in this (start on page 6): https://www.scribd.com/document/326903960/Feynman-Thinking

According to Feyman's experiment, above, it turns out there are at least two kinds of people, those that can read while counting in their head, and those that can speak.

I always wondered why empirically derived differences in people, such as the above, was never used much in other studies. It seems that it would be a lot more interesting data point than the typical self-evaluation questionnaire that most psychology studies seem to use.