The other day I was teaching a business writing class with a varied group of people, among them accountants, purchasing agents, and other "numbers" types who came with their coworkers.
At one point, I wanted the attendees to do only half the sentences in an exercise, so I said this: "If you have an even-numbered birthday, do the even-numbered items. If you have an odd-numbered birthday, do the odd-numbered ones."
Usually dividing the group this way works well. But there was silence after I gave the instructions. Then a woman near the front asked, "Day, month, or year?"
I understood what she meant when she added, "We are accountants. You have to be specific."
Had I thought carefully about the audience before I gave the instructions, I would have said, "If your birthday is an even-numbered day--for example, the 2nd or 4th day of the month--do the even-numbered items."
Know your audience. If you don't, you won't give them the information they need.