Researchers asked 100 fifty-year-olds and 100 twenty-year-olds whether they gave blood. Because nearly twice as many fifty-year-olds as twenty-year-olds reported that they sometimes gave blood, the researchers concluded that, on average, fifty-year-olds are more altruistic than twenty-year-olds. But there is reason for skepticism. Many people hesitate to admit that their behavior does not conform to societal expectations.
The reasoning above calls into question a conclusion drawn from statistical data by
showing that the data are based on an unrepresentative sample
There’s no comment about the background/features of people who were part of the study. What the author’s suggesting is that we can’t trust the “reports.” This doesn’t mean the samples were unrepresentative.
offering an alternative explanation of some of the data
The author suggests that one reason the 50-year-olds might have reported donating blood at a higher rate than the 20-year-olds reported doing so is that many 50-year-olds might not have wanted to admit that they didn’t donate.
showing that one cannot directly observe altruism
The author could believe that it’s possible to directly observe altruism (for ex., we see someone give away a ton of money for free). The author doesn’t show that this is impossible to observe. What she suggeests is that some 50-year-olds might be lying about donating blood.
criticizing the motives of the researchers
The author doesn’t say anything about the motives of the researchers. She simply comments on an alternative explanation for the results of the study.
offering a specific counterexample
The author doesn’t bring up a 50-year-old who didn’t donate, or a 20-year-old who did. (These aren’t “counterexamples” anyway, since you can’t have a counterexample to an average or a general tendency, which already allow for exceptions.)