Mortality Rate – Descriptive Epidemiology

00:00 Now when we talk about mortality rates, there are literally scores of different ways of expressing mortality. Mortality is simply how often people are dying in a population.

00:10 The most basic measurement of mortality is the crude death rate and that's the total number of deaths per year, usually per thousand people, but again the denominator can change depending on the context. The crude death rate for the whole world is currently about eight people per 1000 people per year. That has actually come down, so that's one of the nice things of studying epidemiology, we can tell when things are getting better.

00:34 So in the last few decades and the last few centuries, the crude death rate of the world has actually come down, people are dying less globally. Perinatal mortality rate is the combination of neonatal deaths and fetal death, stillbirths, per thousand births around the year, around the time of birth and maternal mortality rate is when women who are pregnant die due of child bearing. Infant mortality rate is a measurement of children less than one year old, that's the definition of infant, who die per a certain number of live births and similarly child mortality rate is the number of children five years or less per 1000 live births. The last three considerations, child mortality rate, infant mortality rate and maternal mortality rate are particularly important to those of us who study global health, because we use those measurements to determine the overall health of a healthcare system. If a healthcare system can't protect those three vulnerable groups in their population, something probably is seriously wrong. The standardized mortality ratio or SMR was invented as a way to compare different populations, their death rates. And the way it works is we pretend that the group that we are interested in resembles another reference population.

01:49 How do we pretend this? Well we assume that the age distribution of our test population resembles the age distribution of our reference population. This particular is useful when computing mortality rates for certain occupational groups. For example, if I'm wondering if coal miners in the USA are dying more than the regular population, I can look up the death rate amongst coal miners and compare it to the regular population, however, coal miners are typically younger than the general population, so maybe that's not a fair comparison. I adjust the coal miner population statistically to resemble the general population and then I compute my numbers. The SMR tells me the ratio at which the coal miners die more so than the general population. So if it's more than one, we assume the coal miners are dying more than everybody else, if it's less than one, they're dying less than everybody else. So again, the SMR, standardized mortality ratio, is a ratio, a pure number that allows us to compare the death rates in one population to another. Similarly the age-specific mortality rate is when we stratify our population by age groups and compute the death rates for a specific age group. We do this because old people die more so than young people as result of old age, we all understand how that works. So if I'm comparing two populations and one of them has more old people than the other, we expect the population with more old people to die more often than the population with fewer old people. And if I'm trying to tease out whether there is something else going on, like an environmental exposure or a disease, that age factor can get in the way. So by using age-specific mortality rates, I can control or remove the effects of age, so I can investigate more deeply whether or not something else is happening.

03:42 Another important measurement of mortality is the case fatality ratio, again it is a ratio, so it is a proportion. So the case fatality ratio tells us, once you have a disease what is the probability you're going to die from it. So if the case fatality ratio is 1 or 100%, you're always going to die from this disease, it is perfectly fatal, perfectly lethal. If it's quite small, then you'll probably recover from the disease. So it's given as the number of deaths from a given disease, divided by the total number of people who contract that disease. Clearly if you stop and think about it, the CFR will change as our ability to deal with certain diseases improves or decreases. For example, Ebola was known for a long time as having one of the highest CFRs that we knew about, in the last few years as our experience with Ebola got better, we've managed to reduce the CFR of Ebola from about 90% down to 60 or 70%, depending upon the population we're looking at and that's entirely due to medical innovation and experience clinically. So CFR is not hardwired in the disease, it depends on our scientific and medical expertise and how to deal with that disease. For example, the CFR for pneumonia in the USA in 2009 was based on 163 deaths out of about 35,000 cases, that's about 0,45% that is quite low. So we know that if you get pneumonia in the USA, you'll probably not die from it. On the other hand, another measurement of mortality is the proportional mortality ratio, or PMR and this tells us how much a certain disease is responsible for all the deaths in a population. To get it, we take the number of deaths from the disease you care about and divide it by all the deaths in a certain population. For example, the PMR for influenza and pneumonia in the USA is about 62,000 deaths over 2 million deaths total, that gives us about 2,56%. Let's compare those two numbers, 0,45% CFR for pneumonia, 2,56% PMR for pneumonia and influenza, this tells us that if you're going to get one of these diseases, your chances of survival are quite good, but the disease constitutes a fairly robust proportion of the total deaths in the population, 2,56%, I would say is a fairly robust estimate.