On covering one's mouth (when yawning)

A simple observational study recently found that 67.5% of medical students attending morning conference did not cover their mouth while yawning.

The data are still with the statisticians, who raised several problems. Could the extreme boredom of attending these conferences have induced excessive yawning and biased the results? Were genders and races adequately represented in the study? Can the results be extrapolated to livelier conferences? Was there a …