A medical researcher is exploring treatments for a rare disease. Since the disease is not common, she has only been able to locate five patients to participate in a research trial to determine the effectiveness of a new drug treatment.

Which statement BEST summarizes the limitations of this study?

A) The research is probably not valid since the disease is rare.

B) The study would be more valid if additional drugs and treatments could be included. C) Since the study does not randomly assign patients to the treatments, it is invalid. D) The researcher will have less confidence in her conclusions due to the small sample size.

Let's go through each of our options.

For A, the rarity of something has no effect on its validity.

For B, the study wouldn't be more valid, as you should only be testing one variable at a time (in the case, the medicine).

For C, nothing in your experiments should ever be random. And if I'm correctly interpreting it, the question means that even patients who aren't sick would be receiving the treatment, which serves no purpose at all.

Therefore, we know that D must be the correct answer, via process of elimination. Smaller sample sizes are not ideal for experiments, as they require a great deal of extrapolation to see more specific results on a larger scale.