A neurologist friend once told me the best advice that he gave new interns was to practice saying the words "I don't know". I think the same is true for people in public relations. If you don't know, say so. Trying to spin a story when you don't have the facts can only make you look ignorant (at best) and dishonest (at worst), it's far better to just admit that you don't have the information. If you think that the information is important, you might offer to find the answers or even conduct a study. You can also direct the person asking the questions to some resource where they might be able to find the answers.