The difficultness of the daily tasks appears to be reliably indicated by the tendency to yawn.

If you have a physics degree, I think you will be more powerful because of the difficultness.

Clearly, then, our ignorance is due not to lack of appreciation of the scientific value of primate research but instead to its difficultness and costliness.

This excellent showing for the control series wholly justifies the comparison of problems 2 and 3 as to difficultness, made above.