"You can't just plot a single predictor against the sample outcome and expect the plot to be particularly revealing in multiple regression"
Isn't this, effectively, what they are doing when they say they have found an optimum number of "working hours" for one to work to maximize cognitive indicators?
I'm not even quibbling whether they're demonstrating causality versus correlation. The problem is that their result and the number that they found are likely artifacts of the method of the analysis, and the choice to include only linear and quadratic terms.
Isn't this, effectively, what they are doing when they say they have found an optimum number of "working hours" for one to work to maximize cognitive indicators?
I'm not even quibbling whether they're demonstrating causality versus correlation. The problem is that their result and the number that they found are likely artifacts of the method of the analysis, and the choice to include only linear and quadratic terms.