Im really confused on this problem.

Question

A company wants to test the effectiveness of a new teeth whitening strip. The company identifies 500 people to participate in an experiment. The individuals who have never used teeth whitening strips use the new teeth whitening strips. The individuals who regularly use teeth whitening strips use their usual brand. After 4 weeks, the individuals using their regular brand of teeth whitening strips saw significantly less change than those who used the new teeth whitening strips.

Identify a potential problem, if any, with the experimental design.

A)There are no problems with the experimental design.

B) The sample size is not large enough to produce valid results.

C) The groups are not similar.

D) Not enough time passed for valid results to be measured.



Identify a way, if any, to improve the design.

A) The design cannot be improved.

B) The subjects must be randomly assigned to each group.

C) The company should wait longer before results are measured.

D) The sample size must be larger.

Answer 1

They are comparing people who have never used teeth whitener with those who use it regularly. It seems reasonable to me that when you first start using teeth whitener that you would see a big difference at the start and then (as your teeth get whiter) you would see less and less difference as time goes by.

So, the answer to the first problem would be "C) The groups are not similar."

For question 2, I would first "B) The subjects must be randomly assigned to each group." then I would do a comparison of "no teeth whitener" or a placebo, and some additional teeth whiteners so you can compare the effectiveness of yours.