# Missing information anxiety

A recurring theme in math is that you may not need to do what it looks like you need to do. There may be a shortcut to where you want to go. A special case of this is that you may not need all the information that you think you need.

For example, if you need to know the last digit of a×b, it might seem you need to know a and b so you can multiply them together. But you only have to know the last digits of a and b. In fact, if one of the last digits is 0, that’s all you need to know.

As a math consultant, I often tell clients they don’t need information that they think they need. That news may come as a relief, or it may cause anxiety. I may tell a client, for instance, that missing data cannot change a conclusion, so it’s not worth waiting for. Whether that brings relief or anxiety depends on whether they believe me.

There’s a physics demonstration where you have a heavy ball on a long cable. You pull back the ball like a pendulum and let it touch your nose Then let the ball go and stand still. If you’re convinced of the physical laws governing the motion of the ball, you can stand there without flinching. You know that just as it left your nose with zero velocity, it will return with zero velocity. In fact, because of friction, it won’t quite return to your nose. But if you’re not fully convinced of this explanation, you’ll be afraid that a wrecking ball is about to smash your face, and understandably so.

When you tell clients that they don’t need something they think they need, it may come across as if you’re asking them to stand still as a wrecking ball swings toward their face. It’s not enough to be correct; you need to be persuasive as well. Examples help. As with the physics demo, you can put your own own face on the line before asking them to do the same.