Game 18 · Public health and research practice
Which population is really sicker?
Incidence and prevalence, mortality rates, and direct and indirect standardisation.
Read with: Risk and survival, Bias and validity14 DNB questions
Play
Decide which population is really sicker
Some rounds give two made-up populations with different age structures: judge them from the crude rates, then estimate their standardised rates. Others ask for a standardised mortality ratio, or for the new prevalence after a cure or a life-prolonging drug, shown as water in a bathtub. A few rounds test the definitions of IMR, MMR and the case-fatality rate. You have three lives.
Loading the first round…
Comparing death rates
Why crude rates mislead, and how standardisation fixes it
The crude death rate is the number of deaths in a year ÷ the mid-year population, usually per 1,000. It mixes the death rate at each age with the age structure of the population. Death rates are much higher in old age, so a population with more old people can have a higher crude death rate even when its death rate at every age is lower. Age is then linked both to which population people live in and to their chance of dying, so it confounds the comparison.
Directly standardised rate = Σ (standard population in age group × age-specific rate) ÷ total standard population
SMR = observed deaths ÷ expected deaths, expected = Σ (study population in age group × reference rate)
- age-specific rate
- deaths in an age group ÷ people in that age group
- standard population
- one chosen age structure applied to every population being compared
- reference rate
- age-specific rates of a larger population, such as the state
| Feature | Direct standardisation | Indirect standardisation |
|---|---|---|
| Needs | Age-specific rates in each study population, and a standard population | Total deaths and the age structure of the study population, and reference age-specific rates |
| Result | Age-standardised rate, for example per 1,000 | Standardised mortality ratio: observed ÷ expected |
| Use when | Each age group has enough deaths for a stable rate | Age groups are small, or deaths by age are not known |
| Comparing populations | Standardised rates can be compared with each other | Each SMR compares one population with the reference |
A standardised rate is useful only for comparison, and its value depends on the standard population chosen.
Incidence and prevalence
The bathtub: inflow, level and drain
Incidence counts new cases among people at risk over a period. Prevalence counts everyone who has the disease at a given time, old and new cases. Think of a bathtub. New cases flow in from the tap. The water level is the number of people ill now. Cases leave through the drain when they recover or die.
Prevalence ≈ incidence rate × average duration
This holds in a steady state and for an uncommon disease. The exact steady-state relation is P ÷ (1 − P) = I × D (Pearce, 2004). A treatment that keeps patients alive without curing them lengthens duration, so prevalence rises even when incidence does not change: insulin in type 1 diabetes and antiretroviral therapy in HIV are examples. A faster cure, or a higher death rate, shortens duration and lowers prevalence.
Mortality indicators
Definitions examiners ask for
| Indicator | Numerator | Denominator |
|---|---|---|
| Crude death rate | Deaths during the year | Mid-year population, × 1,000 |
| Infant mortality rate (IMR) | Deaths under one year of age during the year | Live births during the year, × 1,000 |
| Neonatal mortality rate | Deaths in the first 28 completed days of life | Live births, × 1,000 |
| Perinatal mortality rate (SRS) | Still births + infant deaths under 7 days | Live births + still births, × 1,000 |
| Maternal mortality ratio (MMR) | Maternal deaths during a period | Live births in the same period, × 100,000 |
| Maternal mortality rate (SRS) | Maternal deaths to women aged 15–49 | Women aged 15–49, × 1,00,000 (per lakh) |
| Case-fatality rate | Deaths from the condition among the cases | People with the condition, × 100 |
A maternal death, as defined by WHO, is the death of a woman while pregnant or within 42 days of termination of pregnancy, irrespective of the duration and site of the pregnancy, from any cause related to or aggravated by the pregnancy or its management, but not from accidental or incidental causes. The case-fatality rate measures how severe a condition is. It is a proportion, not a true rate.
Current figures for India from the Sample Registration System: IMR 27 per 1,000 live births in 2021 (SRS Statistical Report 2021), and MMR 88 per 100,000 live births for 2021–23 (SRS special bulletin on maternal mortality).
Incidence, prevalence, standardisation and life tables are worked through in the risk and survival chapter.
Writing the answer
How to write this in a 10-mark DNB answer
- Define the rate asked for with its numerator, denominator and multiplier, and name the source of the definition (WHO or SRS).
- Give the current Indian figure with its year and source, and one line on why the indicator matters.
- For standardisation, draw a three-row age table for two populations, show the crude rates, then work the direct method with a standard population.
- Show indirect standardisation as observed ÷ expected deaths, and state when it is preferred.
- List the limitations: the standardised rate depends on the standard chosen, and SMRs of different populations are not strictly comparable.
- For incidence and prevalence, draw the bathtub and write prevalence ≈ incidence × duration, with one example of a drug that raises prevalence.
The one thing to remember
Before comparing two death rates, ask whether the populations have the same age structure. If not, standardise.
Past papers
DNB questions this game helps you answer
Questions from NBE theory papers for DNB, DrNB and Diploma courses, most often asked first. The tag after each question is the specialty and the year it was set.
- ×2Define Infant Mortality Rate (IMR).
- ×1Define maternal mortality rate and current rate in India. [2]
- ×1Define perinatal mortality rate and current rate in India. [2]
- ×1What is the definition of infant mortality rate (IMR)?
- ×1Standardized mortality ratio.
- ×1What is a Standardized Mortality Ratio? What are the limitations of using this ratio to compare the performance of Intensive Care Units?
- ×1Define infant mortality rate.
- ×1Standardized Mortality Ratio (SMR).
- ×1Define infant mortality rate (IMR).
- ×1Explain standardized mortality rates with example.
- ×1What are adjusted or standardized rates?
- ×1What is Infant Mortality Rate?
- ×1Infant Mortality Rate is an important indicator of health improvement. Explain.
- ×1Infant Mortality Rate (IMR)
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