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.