A researcher would like to examine how the chemical tryptophan, contained in foods such as turkey, can affect mental alertness. A sample of n = 16 college students is obtained and each student's performance on a familiar video game is measured before and after eating a traditional Thanksgiving dinner including roast turkey. The average score dropped by M = 9 points after the meal with SS = 1200 for the difference scores. a. Is there is significant difference in performance before eating versus after eating? Use a two-tailed test with = .05. b. Compute r ² to measure the size of the effect.