Descriptive Epidemiology: Introduction

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.