00:01
Hello and welcome to epidemiology. If I were
to ask you, how would you describe the burden
of disease in your community, or the presence
of a disease or diseases in a population,
would you know what to do, what numbers to
use, even what questions to ask, how would
you even start that conversation? Well today
that's what we're going to do, we're going
to look at how we describe the presence of
disease in a given population, using classic
epidemiological measures. So you're going
to understand the various measures of morbidity,
including most famously incidence and prevalence.
You're going to understand our measurements
of mortality and you're going to understand
how to read a population pyramid, which is
one convenient, easily digestible way of visually
appraising a demographic distribution of individuals
in a given population.
00:49
So let's go back to my original question,
if you're trying to describe the presence
of a disease in your community, how would
you do it? What questions would you ask? Would
you care about who gets the disease? Would
you care about how many people get it in a
given time period like a year? Would you care
about how many are dying of it or your chances
of dying of it if you were to catch that disease
or maybe you care about of all the things
that kill people in this population, how many
died of this particular disease? Each one
of those questions can be summarized using
a different kind of measure and each one tells
a little bit different, a little bit of wisdom
that's separate from each other, that altogether
they give a comprehensive sense of how that
disease is making its way through a given
population. So in descriptive epidemiology,
we're seeking to summarize some of the conditions
and characteristics of a disease, based on
person, place and time. And this is distinct
from measuring associations between causal
factors, that's what we do in analytical epidemiology.
01:52
In descriptive epidemiology we're looking
for disease burden, in analytical epidemiology
we're looking for associations between variables.
So to summarize what we're going to do today,
we're going to measure incidence, two kinds
of incidence, accumulative and density.
02:07
Incidence is the proportion of new cases of
a disease in a population. Now there are a
couple of important concepts there, the first
is that the cases must be new and the second
is we care about in the denominator how many
people are at risk of getting that disease.
02:21
That means if you already have the disease,
you are not at risk for it or if you're immune
to that disease, you're not at risk for it.
Also, let’s say we're not looking at disease,
we can measure incidence for a variety of
conditions like pregnancy for example. Who
gets pregnant? Women get pregnant, men don't
get pregnant, so in our denominator we don't
care about the men, we only care about the
women in that particular instance. So again
the important concepts there are the newness
and the fact that our denominator are people
at risk.
02:49
Prevalence is another concept and the lay
person commonly conflates prevalence and incidence,
but they're distinct concepts, where incidence
is new cases, prevalence is proportion of
all cases, so the number of all cases of a
disease divided by the number of people in
a population. In mortality rates, we're looking
at the rate at which people are dying in a
given population and there are a couple of
examples of mortality rates that we are going
to explore. One of them is a case fatality
ratio, and that's the idea that once you're
infected with a disease, what's the probability
that you are going to die of that disease.
03:25
We'll look at as well the proportional mortality
ratio and that's the idea that of all the
diseases that are killing people, how much
of that death can be due to one particular
disease of interest. And lastly we will look at
the population pyramid, which is a visual
way of depicting the age distribution of a
population.