So these are two different types of quantities.

Prevalence is a proportion,

its the easy one to calculate actually.

So we have 1,000 people in a population or a

village or you have a sample survey of 1,000 people.

And a 100 of them have an illness so you just make a numerator out

of 100 and the 1000 is the denominator,

and the prevalence of that disorder is 10%.

And this is important because it estimates the

burden of the disorder better then anything anyway.

If you were trying to figure

out about demand for services, If everyone in that...

Illness category needed services, then that would

help you estimate how many, what types of

services, how, how many services, the quantity

of services you would have to plan for.

So, prevalence is a proportion.

It's the proportion in the population with the illness.

That's the easy one. Incidence is a little trickier.

Incidence is the rate at which new cases form in the population.

So, it's sometimes referred to as a rate per

time, and it's not as good an estimate of burden.

But we think it estimates the force of morbidity.

So when I say the force of morbidity.

The force of morbidity is like a, a force, like gravity, pushing a ball downhill.

Except this force is pushing the disease

in the population it's creating the disease.

And so we will be trying to estimate where

the force morbidity is high and where it is low.

Where it is high,

we'll think gosh maybe we can locate the cause of that high force of morbidity.

And so incidence is used for ideologic studies, and trying to

understand the causes of disorders so we have to define the population.

lets take a thousand people again, and you have to define a cohort.

So in those thousand people, it could be

that 100 people have already had the disorder.

Well, they're not at risk for new onset

of the disorder. And this is oriented toward new cases.

So now, we have 900 people who have never had the disorder.

They are the so called risk set.

And now we're going to follow those people, we might follow them for one year.

So we would then say we have 900 person years.

And that would be our denominator, and then we estimate the number of new cases.

If we had, for example, I'm making this easy, 90 new cases, it would be 90

out of 900.

It would be 10%, and that would be if it occurred in one

year, it would be 10% incidence, and that would be an annual incidence.

But you can see, if we follow 900 of the people for

two years and that would be 1800 person years for the denominator.

And we would count the number of new cases might

be 90 and that case, the incidence would be 5%.

So, it's a little trickier but, and I'm going to show

you data for prevalence and incidence as we go along.