Randomization at work. To demonstrate how randomization reduces confounding, consider the following situation. A nutrition experimenter intends to compare the weight gain of newly weaned male rats fed Diet A with that of rats fed Diet B. To do this, she will feed each diet to 10 rats. She has available 10 rats of genetic strain 1 and 10 of strain 2. Strain 1 is more vigorous, so if the 10 rats of strain 1 were fed Diet A, the effects of strain and diet would be confounded, and the experiment would be biased in favor of Diet A.

(a) Label the rats 00, 01, . . . , 19. Use Table A to assign 10 rats to Diet A. Do this four times, using different parts of the table, and write down the four groups assigned to Diet A.

(b) Unknown to the experimenter, the rats labeled 00, 02, 04, 06, 08, 10, 12, 14, 16, and 18 are the 10 strain 1 rats. How many of these rats were in each of the four Diet A groups that you generated? What was the average number of strain 1 rats assigned to Diet A?

EXPLORING THE WEB AppendixLO1