A researcher studying dietary habits in a country surveys only individuals who volunteer after seeing flyers in health food stores. The researcher then claims the findings accurately describe the eating habits of the entire country's population. This approach introduces a significant methodological problem that mirrors a primary criticism of the influential mid-20th century studies on human sexuality. What is this critical flaw?

A researcher in the mid-20th century conducted a large-scale study on private adult behaviors by recruiting participants primarily from volunteer organizations, prisons, and social clubs. Critics later argued that the findings could not be generalized to the entire population. Which of the following principles of sound research methodology does this specific criticism address?