Statistics presents information in a useful manner that is easily understood by people. It's difficult for some a person to comprehend numbers such as 2,000 out of 6,000, but if you told him 1 out of 3 instead, then he can relate better. It gives the same information. Another useful tool is to similarly express the ratio as an even number. This allows easy comparisons to be drawn between different-sized groups. Saying 2,000 out of 6,000 in one group and 9,990 out of 15,000 in another group makes comparisons difficult, but saying 333 out of 1,000 in the first group and 666 out of 1,000 in the second group allows for easy comparisons: group two has twice the prevalence rate.
Look at the total number of occurrences and the total population size. As an example, suppose you wanted to find out the annual rate of muggings per thousand people. You might find that there are 10,000 muggings every year in a city whose population is 250,000.
Divide the population size by one thousand. In the example, 250,000 divided by 1,000 equals 250, which is called the quotient, the result of division.
Divide the number of occurrences by the previous quotient. In the example, 10,000 divided by 250 equals 40.
Interpret the results as this last quotient per thousand. Since there are technically 250 groups of 1,000 people, and on average in each of these 250 groups there are 40 muggings, then you know the prevalence rate is 40 muggings per 1,000 people.