Confound

And the brother of Jared being a large and mighty man, and a man highly favored of the Lord, Jared, his brother, said unto him: Cry unto the Lord, that he will not confound us that we may not understand our words.

1651 (Latin edition 1642), Thomas Hobbes, De Cive (Latin title) Philosophicall Rudiments Concerning Government and Society (English), Hey who lesse seriously consider the force of words, doe sometimes confound Law with Counsell, sometimes with Covenant, sometimes with Right. They confound Law with Counsell, who think, that it is the duty of Monarchs not onely to give ear to their Counsellours, but also to obey them, as though it were in vaine to take Counsell, unlesse it were also followed.

The participants certainly differ in how their practice is distributed (1, 2, or 3 days), but they also differ in how much total practice they get (3, 6, or 9 hours). This is a perfect example of a confound—it is impossible to tell if the results are due to one factor (distribution of practice) or the other (total practice hours); the two factors covary perfectly.