A nutrition blogger claims that because 90 percent of her readers who follow a gluten-free diet say they feel healthier, a gluten-free diet must improve health for most people.
The blogger's reasoning is most vulnerable to criticism because it
- does not specify precisely how long each reader has remained on the gluten-free diet being evaluated
- relies on a self-selected sample that is unlikely to represent the health effects on the general population (correct answer)
- fails to indicate whether respondents recorded their answers anonymously or provided identifying information
- does not explain whether readers combined the diet with other lifestyle changes such as exercise or meditation
Explanation: When you encounter critical reasoning questions that ask about flawed arguments, look for gaps between the evidence presented and the conclusion drawn. Here, the blogger concludes that gluten-free diets "must improve health for most people" based on feedback from her readers. The fundamental flaw is sampling bias. The blogger's readers who follow gluten-free diets represent a self-selected group—people who chose to read her blog and chose to adopt this diet. This sample is inherently biased because it excludes the broader population who might not respond well to gluten-free diets, don't read nutrition blogs, or abandoned the diet due to poor results. You can't generalize from such a narrow, self-selecting group to "most people." Let's examine why the other options miss the mark: Choice A about diet duration is irrelevant—even if we knew the timeframe, the sampling problem would remain. Choice C regarding anonymity doesn't address the core logical flaw; anonymous responses might actually be more honest. Choice D about other lifestyle changes, while potentially relevant, isn't the primary weakness since the sample itself is fundamentally unrepresentative. The key insight is that people who actively seek out and stick with a particular diet are likely to be predisposed to positive outcomes, making their experiences poor predictors for the general population. Strategy tip: In GMAT critical reasoning, when an argument generalizes from a specific group to a broader population, immediately ask yourself: "Is this sample representative?" Self-selected groups—whether blog readers, survey volunteers, or product users—are classic red flags for sampling bias.