We all know that a percent is one part per 100, but things can get confusing when we talk about percentages without putting them in context. For example, a recent news report warned that "white women who get five or more blistering sunburns before the age of 20 have an 80 percent increased risk for melanoma" [source: HealthDay]. Eighty percent sounds huge, but since the American Cancer Society estimates the risk of developing melanoma at around 2 percent for women, an 80 percent increase puts the new risk at about 3.6 percent. So the absolute risk increases by 1.6 percentage points (or about 1.6 cases per 100 people), but the 80 percent jump in relative risk (i.e., risk compared with others in the study) is sure to get more headlines.
Marketers are masters at using percentages to sell products and ideas ("30 percent fewer calories!" "10% whiter!"). But quoting percentages without understanding where they came from can lead to all kinds of misinformation. Case in point: You've probably heard the myth that 50 percent of U.S. marriages end in divorce. The National Center for Health Statistics arrives at that figure by comparing the annual marriage rate per 1,000 people to the annual divorce rate [source: Hurley]. But since the people divorcing in any given year are not the same people who married that year, looking at the numbers for any one year doesn't really tell us anything.
For one more scientific word you may be using wrong, read on!