They did ask how the players felt, because the GP has a good point, and included it in the analysis: "3.4.3. Enjoyment item After participants in each conditi

They did ask how the players felt, because the GP has a good point, and included it in the analysis: "3.4.3. Enjoyment item After participants in each condition finished their 8 h of gameplay and before the posttest battery of tests was administered, we presented an item to players concerning their enjoyment of the assigned game. This was a single self-reported response to the statement: “I enjoyed playing (Portal 2 or Lumosity).” The 5-point Likert scale ranged from 1 (strongly disagree) to 5 (strongly agree)." "The measures were controlled for player enjoyment in all models (when significant) to rule out any effect enjoyment might have in learning gains. That is, based on participants' responses to the statement about enjoying their assigned game, for Portal 2 participants, enjoyment M = 4.32; SD = 0.93, while for the Lumosity participants, M = 3.50; SD = 1.05. The difference between the two groups' enjoyment is significant, with a strong advantage for the Portal 2 group: F(1,73) = 12.69; p < .001. Cohen's d = .83, which is a large effect size." "Enjoyment was a significant covariate in the VSNA ANCOVA model"