Critical Reasoning (CR) | Re: Journalist: A recent study showed that people who drink three cups

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. Clearly, decaffeinated coffee must contain something that damages connective tissue and that is not present in regular coffee.

Which one of the following would be most useful to know in order to evaluate the journalist's argument?