A ratio or proportion, often expressed as a percentage (per 100), but which may also be expressed per 1,000, per 10,000, per 100,000, or even per million. These “per” numbers are called the “base.” Thus a rate expressed per 100,000 is said to have 100,000 as the base. The base chosen is usually large enough to insure that the rate will be expressed in whole numbers; the more rare the event, the larger the base chosen. A death rate of 7 per 10,000 is easier to understand than a rate of 0.07 percent (although both actually give the same information).