Imagine that a nurse takes a child's temperature. However, the mercury thermometer being used is marked off in intervals of one degree. There is a 98° F mark and a 99° F mark, but nothing in between. The nurse announces that the child's temperature is 99° F. How does someone interpret this information? The answer becomes clear when one has an understanding of significant digits.

The Importance of Precision

One possibility, of course, is that the mercury did lie exactly on the 99°F mark. What are the other possibilities? If the mercury were slightly above or below the 99°F mark (so that his actual temperature was, say, 99.1°F or 98.8°F), the nurse would probably still have recorded it as 99°F. However, if his actual temperature was 98.1°F, the nurse should have recorded it as 98°F. In fact, the temperature would have been recorded as 99° F...