In the book Faster: The Acceleration of Just About Everything, author James Gleick reveals some interesting facts about the original research that produced the Type A categorization.

Gleick tells how the research was flawed without a real control group and the candidates were chosen from people who already fit the profile. If they smoked, checked their watch numerous times during the interview, or were in a high stress job, they were chosen for the test, thereby skewing the findings. Also, the Type B personalities were never clearly defined either. The people who were found to be Type B were mainly people who were not like Type A. If they were not highly stressed or seemingly didn't care much about time they were considered Type B.

Finally, according to Gleick, while popular media, management theory speakers, and stress management gurus picked up on the Type A and Type B categories in the original research, they neglected to mention a third group, the Type C personality. This group was found to have no concern at all about time or stress. The group was composed of a half-dozen blind unemployed guys from San Francisco where the original research was done.