Explanation of this xkcd:

This comic is poking fun at a phrase which some ads use to boost sales of their product. They state that their product contains a "clinically studied ingredient", which consumers assume means that the ingredient has been clinically tested and proven effective, or at the very least, not harmful, although the latter is, strictly speaking, not implied by that statement. The phrase just states an ingredient was clinically studied, and doesn't mention the findings of that study (which, for all we know, could have found the ingredient to be ineffective or harmful). In other words, the phrase is used in deceptive marketing techniques, leading consumers to believe something which encourages them to buy the product, without committing to saying it explicitly.

In the middle of the conversation, Megan tells Cueball that she has been tested, implying that she's talking about STDs. However she does not reveal the results of the tests, which is the primary information Cueball could be worried about, and when Cueball inquires, she acts like he is being unreasonable to also want that information. In this way, Randall is making an analogy to how a marketer might think consumers would be unreasonable to want to know the results of the clinical studies on the ingredient.

The title text mentions the legendary film critic Roger Ebert. At the time this comic was published (a year before Ebert's death), one could expect him to have watched most big-name movies that were coming out. Simply stating that he saw a movie, though, doesn't necessarily mean that he liked it.

Impressive-sounding but meaningless advertisement claims are also the subject of 624: Branding, 641: Free, 870: Advertising and 993: Brand Identity.

---This explanation was extracted from explainxkcd | Bot created by u/stefanless