Measuring a population's health: epidemiology, mortality, morbidity, incidence and prevalence

Epidemiology is the study of patterns and causes of health and disease across populations. The definition is easy; the reason it exists is the part worth understanding.

A doctor sees one patient at a time and can say what is wrong with that person. No amount of that work answers the questions a health system actually has to answer: is this getting worse, who is it worst for, and would money spent here save more life than money spent there. Those are questions about groups, and they can only be answered by counting.

Epidemiology answers four questions in order, and the order matters because each one narrows the next:

• What is happening? How much of this condition is there.
• To whom? Which groups, by age, sex, location, income, background.
• Is it changing? Compared with last year and with ten years ago.
• Why, and what works? Which patterns suggest causes, and which interventions moved the numbers.

In Australia the main source you are expected to know is Australia's Health, published by the Australian Institute of Health and Welfare. It, the Australian Bureau of Statistics, and state health department reports are where the tables and graphs in your exam will come from.

Mortality counts deaths, usually as a rate per 100,000 so that populations of different sizes can be compared. It is the most reliably collected health measure there is, because deaths are registered by law. Its weakness is that it says nothing about anyone still alive.

Infant mortality counts deaths before age one, per 1,000 live births. It gets its own measure because it responds sharply to things that matter far beyond infants — maternal health, nutrition, housing, sanitation and access to care — which makes it one of the most sensitive indicators of a population's overall conditions.

Morbidity counts illness, disease and injury among the living. It is where most of the burden of health actually sits in a country like Australia, where chronic conditions are managed over decades rather than being fatal quickly. It is also harder to measure than mortality, because it depends on people presenting, being diagnosed and being recorded — which is a real limitation and a frequent exam point.

Incidence counts new cases in a period. Prevalence counts all cases existing at a time. Get this pair right and a whole class of questions becomes straightforward.

The dot point does not ask you to memorise figures. It asks you to use tables and graphs to explain health status, and the examinable skill is reading one you have never seen before. Ask these five questions every time, in order:

• 1. What is the unit? A count, a percentage, or a rate — and if a rate, per how many. A number without its unit is not evidence.
• 2. Who is in it? The population, the age range, the years covered. Half of all misreadings happen here.
• 3. Is it age-standardised? If it is, differences between groups are not explained by one group being older. If it is not, they might be entirely.
• 4. Which way is it moving? One year is a position; several years is a trend, and only the trend supports a claim about improvement.
• 5. What is missing? Which groups are not shown separately, and what the measure cannot see.

Question 3 deserves a sentence of its own, because it is the one students skip. Age standardisation adjusts rates so that populations with different age profiles can be compared fairly. Without it, an area full of retirees will show more cancer than a university suburb, and the finding tells you about ages rather than about health.

The verb in this dot point is discuss, which means the limitations are part of the content, not a criticism bolted on at the end. Four limitations are worth knowing well.

• An average hides its distribution. A national figure that improves can conceal a group whose figure is getting worse, because the majority moves the average. Any claim that “Australia's health is improving” is a claim about a mean, and Lesson 5 is about who sits away from it.
• Morbidity data depends on being counted. A condition is recorded when someone presents, is diagnosed and is entered into a system. Groups who cannot easily reach a service therefore appear healthier in the data than they are, which is exactly backwards.
• These measures count disease, not health. Every measure in this lesson counts something going wrong. None of them measures wellbeing, capability, or the social and mental dimensions from Lesson 2 — so a purely epidemiological picture of health is a biomedical one, in the sense Lesson 1 defined.
• Numbers arrive late. Collection, checking and publication take time, so today's report describes a situation that has already moved.

No single measure describes health status. The skill the dot point is really asking for is reading several together, because each one covers a blind spot in another.

Work through Table 1 above. Incidence falls year on year, so fewer people are newly acquiring the condition — prevention is working. Mortality halves, so those who have it are surviving longer — treatment is working. Prevalence rises by 35 per cent, and taken on its own that reads as a crisis. Read with the other two columns, the rise is the consequence of the success: people who would have died are alive and are still counted.

That is the reasoning an examiner is looking for, and notice what it required. It required knowing what each measure counts, noticing that the three columns disagree, and resolving the disagreement rather than picking whichever number suited the argument. The last part is the one that separates a Band 6 answer from a Band 4 one.

The same pattern appears in real Australian data. Falling death rates from a chronic condition alongside rising prevalence of that condition is one of the most common shapes in Australia's Health, and it usually means people are living with something rather than dying of it — which shifts the burden from mortality to morbidity, and changes what the health system needs to provide.

Run the five questions from Card 06 over Table 1, writing one sentence for each. Then answer the three below.

Go to the Australian Institute of Health and Welfare's Australia's Health and find one table or graph on a condition that interests you. Do not choose the first one you see; choose one that reports more than a single year.

• Record the source and the year of publication.
• Run the five questions from Card 06 over it and write one sentence each.
• Write a three-sentence statement of what it shows about the health status of Australians, using at least two of the five measures by name.
• Write one sentence naming something it does not show that you would want to know.

Compare the figures you find with the constructed ones in Table 1. If they differ substantially, that is expected and is the reason the table above is labelled as practice data rather than presented as fact.