## Lies, D*** Lies, and Percentages

### Finally, there are a couple of things you should know about calculating and interpreting percentages. First, something can't decrease by more than 100%. Once 100% of something is gone, there isn't anything left. Never write that a price or anything else decreased by 150%. It's impossible*. Second, when you are reading about medical, political, or financial news it is important to understand that big percentages can mean small overall increases or decreases. For example, an article that reports a 50% increase in the rate of a rare disease may be telling you that instead of 1 in 100,000 people getting floogety flork disease every year, now 1.5 people in 100,000 get the disease every year. A 50% increase sounds a lot scarier than the increase in raw numbers. Percentages aren't always misleading, but it's something to watch out for.

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References

1. percent. Dictionary.com. Dictionary.com Unabridged (v 1.1). Random House, Inc., http://dictionary.reference.com/browse/percent (accessed April 3, 2008).
2. Garner, B. Garner's Modern American Usage. Oxford: Oxford University Press, 2003, p. 598.
3. Wikipedia Contributors. ed. Nygaard, G. Wikipedia: Manual of Style, http://en.wikipedia.org/wiki/Wikipedia:Manual_of_Style_(spelling) (accessed April 3, 2008).
4. Brians, P. "percent, per-cent," Common Errors in English Usage, http://www.wsu.edu/~brians/errors/percent1.html (accessed April 3, 2008).
5. percent. The American Heritage Book of English Usage. Houghton Mifflin Company, 1996, http://www.bartleby.com/64/C003/0227.html (accessed April 3, 2008).
6. percentage. Dictionary.com. The American Heritage® Dictionary of the English Language, Fourth Edition. Houghton Mifflin Company, 2004, http://dictionary.reference.com/browse/percentage (accessed April 3, 2008).
7. "Numbers," The Chicago Manual of Style, Chicago: The University of Chicago Press, 2006, section 9 (accessed March 25, 2008).
8. Aaron, J. The Little, Brown Essential Handbook. New York: Pearson Education, 2006, p. 101.
9. Goldstein, N., ed. The Associated Press Stylebook and Libel Manual. Reading: Perseus Books, 1998, p. 156.
10. Lutz, G. and Stevenson, D. Grammar Desk Reference. Cincinnati: Writer's Digest Books, 2005, p. 321.
11. Burchfield, R.W., ed. The New Fowler's Modern English Usage. Third edition. New York: Oxford, 1996, p. 535.

* Sigh. I should know better than to use absolute words like impossible. People have almost convinced me that when a value can become negative, it is possible for the value to decrease by more than 100%. What do you think? Join the discussion in the comments.