Question

a company wants to test the effectiveness of a new acne cream. the company recruits 50 girls ages 13 to 17 who have acne. the subjects are randomly assigned into two groups. one group is given the acne cream and the other is given a placebo that looks exactly like the acne cream. both groups apply the cream daily for two months. facial photos are taken at the beginning and end of the treatment to compare results. complete parts (a) through (c)
(a) identify the experimental units and treatments used in this experiment. choose the correct answer below.
a. the experimental units are the 50 girls ages 13 to 17 who have acne. the treatment is the group assignments.
b. the experimental unit is the new cream used to treat acne. the treatments are the facial photos.
c. the experimental units are the 50 girls ages 13 to 17 who have acne. the treatment is the new cream used to treat acne.
d. the experimental unit is the new cream used to treat acne. the treatments are the two months over which the cream is applied.
(b) identify a potential problem with the experiment design being used and suggest a way to improve it. choose the correct answer below.
a. only girls ages 13 through 17 were used. to increase validity, use a stratified sample.
b. only one placebo cream was studied. to increase validity, use more creams of different varieties.
c. facial photos were used to compare results. to increase validity, use video recordings to compare results.
d. the cream is used for two months. to increase validity, compare photos immediately after applying the cream.

Explanation:

(a) Experimental units are the entities on which the experiment is performed. Here, it's the 50 girls aged 13 - 17 with acne. Treatment is what is being applied to the experimental units, which is the new acne - cream.
(b) Using only girls aged 13 - 17 limits the generalizability of the results. Using a stratified sample would increase the validity as it would account for more diverse factors.

Answer:

(a) C. The experimental units are the 50 girls ages 13 to 17 who have acne. The treatment is the new cream used to treat acne.
(b) A. Only girls ages 13 through 17 were used. To increase validity, use a stratified sample.