Passage Summary: A journalist claims decaf coffee must have a unique harmful ingredient because decaf drinkers suffer from arthritis at twice the rate of regular coffee drinkers.

Conclusion: Decaffeinated coffee contains a specific substance that causes joint damage which is not found in regular coffee.

Reasoning: A study indicated that individuals who consume three cups of decaf daily are twice as likely to have arthritis as those who consume the same amount of regular coffee.

Analysis: To evaluate this argument, we must determine if the two groups being compared were actually similar to begin with. The journalist assumes the coffee type is the cause, but it is possible that people who already have joint pain or other health issues are more likely to choose decaf over regular coffee. We need to know if there are other differences between decaf and regular coffee drinkers—such as age or lifestyle—that might account for the higher arthritis rates. Look for an answer that asks about the characteristics of the people in the study before they started drinking their respective coffees.