A psychologist studying the effects of positive thinking" on cognitive ability designs the following study: 120 male volunteers are divided into two groups by random assignment. Eighty female volunteers are randomly divided into two groups of 40 by the same method. One group of 60 males and 40 females is asked to write down three things they feel that they are good at doing. The other group of 60 males and 40 females is asked to write down what they ate for breakfast. All 100 subjects then take a SAT Math practice test, and the means for the two groups are compared. Which of the following is a correct description of this study? i. An observational study
ii. A completely randomized experiment
iii. A matched pairs experiment
iv. A stratified random experiment
v. A randomized block experiment