I wa thinking about innumeracy and this example. Someone not knowing how to figure out a percent change in something.

It's a fairly simple concept, but the numbers can sound funny. For example, if there are two people in a restaurant, and four more walk in, the you have an increase of 200%. Three walk out, for a 50% reduction. One walks back in because she forgot her cell-phone, for a 33% increase. She leaves again, for a twenty-five percent decrease in restaurant patrons.

If my salary goes up 50 percent one year, and then down 50 percent the next year, I have a much lower salary than I started with.

!% of people use a certain dangerous drug in the 90s. The next decade 3% are using it. An increase of 200%. It sounds very scary. Or is that only a two percent rise (a rise in percentage, not a "percent change")?

If twice as many people do something, that's an increase of 100%. But I wonder if when people think about a number tripling they think of 200% or 300%? Or a "five-fold" increase in something? What does that mean--five times or 500%?