When calculating utility, what is the concept that measures the true difference in performance between average trained and untrained employees in standard deviation units?

A) the standard deviation of untrained employees in dollars
B) the benefits-cost ratio
C) the effect size
D) return on investment

The concept that measures the true difference in performance between average trained and untrained employees in standard deviation units is the effect size. It is used to determine the practical significance of the training program and measures the magnitude of the difference between the means of the trained and untrained groups. Option A is incorrect because the standard deviation of untrained employees in dollars is not a suitable measure for calculating utility. Option B is incorrect because the benefits-cost ratio only provides a financial measure of the value of the training program. Option D is incorrect because return on investment measures the financial return on a training investment and does not consider the difference in performance between trained and untrained employees.