First we need a bit of vocabulary -- what we call some of the **kinds** of rates or proportions that we commonly talk about in public health. Last time, I said that "rate" and "proportion" basically mean the same thing, and they do; but we tend to use them in slightly different contexts. We can say that:

A “rate” means the proportion of a population which either 1) has some characteristic at a given time, or 2) to which some event occurs during a given period.

In public health, a “population” normally means some defined group of humans – but in statistics, a “population” can consist of anything, and sometimes our units aren’t people – they might be hospitals, or procedures, or laboratory rats, for example.

To compute a proportion, divide the number of “cases” (people with the characteristic) by the total population of interest. If you have 18 people, and 3 of them have nose rings, the nose ring proportion is 3/18=.167=16.7%. As I said last time, all three ways of writing it mean exactly the same thing.*

There are two major kinds of rates:

PREVALENCE

andINCIDENCE

**Prevalence**means the proportion of people who have the characteristic at

**a specific point in time**. So, in our group of 18 people, the prevalence of nose rings is .167, or one out of six if you prefer.

The

**incidence**is the proportion of people who acquired the characteristic

**within a specified period of time**. Usually, it's calculated for a calendar year, but it certainly doesn't have to be! If it turns out that 2 of our 18 people already had nose rings as of January 1, 2009; and 1 of them got a nose ring for the first time during 2009, the

**incidence**of nose rings for 2009 is 1/18=.055555555555555555555 and on to infinity which we would probably round off as .056 or 5.6%. Next time, I'll use 20 people with a prevalence of 5 nose rings and an incidence of 2 so you'll get nice simple fractions and snappy finite decimals. But the idea is the same. And now I'm going to write a dreaded formula!

# of people who

When we write formulas we like to save space by using short symbols in place of long phrases so we might write this as:

R/N=I

where R=number of people

*3/18 happens to be the same as 1/6, and one sixth can't actually be written as a decimal because it would be infinite, so 1.67 or 67% is only a close approximation. Not worth worrying about.

## 2 comments:

Isn't R/N=P ?

number of people who have the thing/ total number of people

I=number of people who get the thing/total

i'm following you so far.

not understanding anony, though.

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