"This is a numerator. Without a denominator, it tells me /n/o/t/h/i/n/g/ very little."

I can't remember where I first heard that (although I have a vague impression that it might have been alt.poly), but it's very useful when evaluating scare stories in the media. "Twenty-eight people have been killed in medical evacuation helicopter crashes in the last two years! The FAA must take action to better regulate these flights!" I'm honestly sorry to hear that, and my sympathy to their families, but how many flights were there? How many people were transported safely? Is this really a crisis? I don't know. I have a numerator, but no denominator.

Related to this, you cannot compare numbers and percentages and expect any kind of valid results. It makes no sense whatsoever to say, "15% of the people in Department A were laid off, but only 2 people in Department B were laid off." (This is an argument that I had, repeatedly, with a lawyer I used to work for.) In many cases, you can't really compare numbers with numbers, even; you need to compare percentages.

ETA: Changed the subject line slightly; redbird pointed out that the difference between "numerator = 0 --> nothing has happened" and "numerator != 0 --> something has happened" can be useful information.