Percent increase

Today I heard an extremely competent financial anchor say that MGM stock was up 250% from its March low. The stock was trading near 8.20, and the March low was 1.81. In other words, the current price was roughly 4.5 times the low.

The correct way to describe this situation is to say that the stock is up 350%. Many people, however, would say that it's up 450%. They forget that the word up indicates the difference between the prices, not the ratio between them. The anchor's error, however, went in the other direction.

My guess is that the anchor received written copy that correctly said that the stock was up 350%. Familiar with the usual error, the anchor assumed that the copywriter had made it. She compensated by subtracting 100 from the correct percentage, arriving at the erroneous 250%.