A consumer magazine surveyed people who had sought a psychologist's help with a personal problem. Of those responding...

Skylar on February 8, 2020

@Maria-Marin, happy to help.

The conclusion of the argument is that "psychological treatment lasting more than 6 months is more effective than shorter-term treatment."

(C) says "patients who feel they are doing well in treatment tend to remain in treatment, while those who are doing poorly tend to quit earlier."

This weakens the argument by suggesting that the sample is unrepresentative of the general population.

If most people who felt that treatment did not make anything better quit early on and most people who felt that treatment was making things significantly better continued with treatment, it follows that patients who are in treatment longest would report feeling the biggest effects. However, this does not necessarily mean that the patients stayed because long-term treatment effectively made things better. Instead, it could be the case that things getting better/patients already feeling that they are doing well is what made them decide to stay.

In other words, the fact that the people who feel positive effects of treatment are the only ones who tend to stay in long-term treatment greatly affects the outcome of the survey. We cannot say that long-term treatment is more effective for everyone, because it tends to be only patients who already felt they were doing well that make up this category. Would the same be true of patients who felt they were not doing well? Therefore, (C) is correct.

Does that make sense? Please let us know if you have any other questions!