0:24

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.

3:03

Here's the prevalence of depression in percent.

It's just a simple percentage from selected surveys.

And there are 42 studies that we reviewed in the book, Public Mental Health.

And there are other studies.

Even as we speak, there are new studies being done.

And I've selected these studies to give you a feeling for the range.

I selected three from the United States, these are all of adults.

And, we have prevalence quantified

in two ways.

We have the one year prevalence, that

means, how many people in the population, what

percentage of the people in the population,

have had the disorder within the prior year.

So, for the, US, epidemiologic, catchment area study 3.5% over

the past year have met the criteria for major depressive disorder.

But we can also estimate what percentage

of the population have never met the criteria

for major depressive disorder over their entire life.

And we call that lifetime prevalence and that's

naturally higher its got more time to run.

And that turns out to be about, 6% in the, in the ECA study.

you can see that the prevalence rates range a fair amount.

I've selected only studies of adults.

And I selected the lowest study I could find,

which was in Taipei, Taiwan, the major city in Taiwan.

And that was

a well-conducted study, but it's only 0.6, that is less than one in 100 people.

Depressive disorder, according to the diagnostic criteria in one year.

And that contrasts with Urdmutria, Russia, which is

an area about 1000 kilometers east of Moscow.

A rural area and there are four rural village there.

And this again is a credible study conducted by this fellow,

[UNKNOWN].

But they found 22% even in one year and 32% lifetime prevalence.

So, you can see, depression varies around the world.

There's lots of variation, but it's big.

Prevalence rates are bigger than many other major illnesses.

4:57

This gives you a feeling.

This is the one year prevalence, the 12

month prior from the World Mental Health Surveys.

And the disorders are a little varying depending on which country

you're looking at.

But you can see, we have about 10% in the United States.

The green is North America.

And then we have the European continent in brown, and African

and Middle East in black, and, and East Asian in red.

And again, we see a relatively low rate of depressive

disorder in East Asia, and a surprisingly low prevalence in Nigeria.

So I think it may be interesting, you will hear from Professor Tull

in a later lecture about the meaning

of depression, and the cultural variation in it.

And that, we'll connect a little bit to

what these state would show lots of variation.

5:44

Now, lets estimate incidence.

So we have to have a longitudinal study.

And I'm going to show you in the rest of this

lecture, several slides about the

Baltimore Epidemiologic Catchment Area followup.

And this particular aspect of the followup is

from 1981, when the first interviews were done,

until 1996, when a followup interview was done.

Just to give you a feeling for the population aspect of it.

We defined an area in Eastern Baltimore of 175,211

adults, then we designed a sampling regimen, which would

sample each individual into a sample with a known probability,

that's probability sampling.

And we ended up with designating 4,238 people for the sample.

But we only were able to persuade

3,481 to, to actually participate in the interview.

So you can see the challenges in this kind of work.

And in the time between 1981 and the followup, 848 of these had died.

And we had trouble locating another 415. That's 16% of the survivors;

they just had moved, and we could not find them.

We tried to find them so we could interview

the same person again, and could not find them.

We located 2,218 and of those, 298 people

said, you know I don't really have time, or

I'm not interested, or I didn't like the interview

last time, or I don't want to do it.

They refuse.

So, this is attrition which happens in a cohort study.

These are the major sources of attrition

in cohort studies in epidemiology death, location

and refusal, but we had 1,920 that were interviewed.

So we're going to construct a cohort to estimate incidence with those 1,920.

And this spreads out the cohort according to sex and age.

And if you look at the first column.

The at risk column, you can see instead of 1,920, we have 1,665 at risk.

That's at

the bottom of the second column.

Well, that's because, some of the 1,920 already

had depressive disorder, so they're not at risk.

Okay so within each sex by age group it list the at risk

population and the number of new cases during that 13 year follow up.

So for 18 to 29 year old males, they were 18 to 29

in 1981, and they were 237 of them and 9 of them over

that period develop an episode of major depressive disorder.

And the right hand column is

the cumulative incidence, that's the proportion.

So, 9 divided by 237 is 3.8%. That's

8:29

And you can see these 71 new cases and the number is

red at the, in the center-left there, that we have 71 new cases.

And during that time there were 23,698 person years of exposure,

among those 1665 people.

We divide 71 by 23,698 and we get 3.0, actually, per thousand per year.

That's the incidence of depressive disorder.

Now one of the things this slide shows is, it shows

the way the incidence rate evolves over the course of life.

We have females in the brown dotted line and males

in the black line.

You can easily see that the annual

incidence per thousand there is averaging about three,

but for females that are age 30 it's nearly eight, it's seven and a half.

That's the peak incidence.

It's interesting that the peak incidence for females is earlier than for males.

You can see the incidence rate for females is about twice as high as for males.

That's standard in the literature.

And now that we understand

incidence as the force of morbidity, we're trying to think, why is it.

That, the rate of the depressive disorder peaks at the age of 30.

And so, that's a clue to the ideology.

That's one of our jobs as epidemiologists, is to develop clues for etiology.

And we're going to wonder, what is happening in the life course

at that period of time that raises risk for depressive disorder?

And we can think back.

It's not inheritance,

because inheritance doesn't change over the course of life.

But stress and loss does change over the course of life.

So we're looking for that possibility.

Now one of the other things is, if you look on the right-hand side there, there's

a trough and the rate of incidents at about age 50, both for males and females.

And then it raises, and I can tell you that when we first

did this analysis We saw that second bump, and we thought, that must be

a mistake in the data.

Maybe we can massage the data somehow and

get rid of that bump, so it'll look smoother.

But we couldn't make the data obey, and it showed the bump.

And this bump now, we're trying to figure out what

is it that's low and rises at the age of 50?

So for example, we're led to think about loss.

After the age of 50, there is more bereavement.

For example, spouses are dying, male spouses

are dying.

But it's also true that we have more,

physical illnesses growing in force at that time.

And I'm going to show you data about that later in the lecture.

That concludes our study of the

prevalence and incidence of major depressive disorder.

If you had to remember something about this, you might remember that

depressive disorder has a prevalence of about 10%, in the Western populations.

And an incidence per year of about three per thousand,

which is less than 1% per year.

And we'll turn next to the natural history of depressive disorder.