Journalist: A recent study showed that people who drink three cups of decaffeinated coffee per day are twice as likely to develop arthritis—inflammation of joints resulting from damage to connective tissue—as those who drink three cups of regular coffee per day.
Conclusion (clearly)
Decaffeinated coffee must contain something that isn't present in regular coffee that damages connective tissue.
Evidence
In a study, people who drink 3 cups of decaf every day were twice as likely to develop arthritis as people who drink 3 cups of regular coffee per day.
Evaluate
Any time the author's conclusion is stating or assuming a causal explanation for the results of a study, we go into Explain Curious Fact mode.
Curious Fact ... How come people who drink 3 cups of decaf are twice as likely to get arthritis, compared to people who drink 3 cups of regular?
Author's Explanation ... There is a special ingredient in decaf, not in regular, that damages connective tissue, which leads to the arthritic inflammation of joints.
We're always supposed to consider 1. Alternate Explanations for the curious fact 2. the plausibility of the Author's Explanation
Any time we're given a correlation between x and y (in this case between decaf and arthritis), there are two automatic alternate explanations we want to consider:
- could it be Reverse Causality? is it possible that the arthritis came first, and since the arthritis made it harder to grasp the coffee mug securely, those people switched to decaf so they wouldn't have the shaky hands a caffeinated person might have? The problem with that storyline here is that the correlation seems to have a built in before/after. It says that the people drinking decaf are more likely to develop arthritis (in the future). If it said they were more likely to have arthritis, then we could definitely consider this reverse causality angle.
- could there be a Third Factor that is the real causal agent, and the two parts of the correlation are just symptoms of that third factor? is it possible that "old age" is the common ingredient? If older people are more likely to drink decaf and more likely to develop arthritis, then that would explain why decaf and arthritis are correlated, but it wouldn't be decaf causing the arthritis.
When it comes to the plausibility of the author's story, she has to assume that the process of decaffeinated coffee involves adding some new ingredient. If it's only removing caffeine, then decaf and regular coffee are chemically the same, other than caffeine, and so her story would make no sense.
Goal
Let's look for answer that either allows us to argue that there's an Alternate Explanation for why decaf is correlated with higher rates of arthritis, or one that deals with the plausibility of the author's storyline that decaf has a special ingredient that damages connective tissue.