Incidence, Prevalence and Mortality
- Junessa Masaya
- Apr 17
- 5 min read
HSC Biology | Study Notes
Incidence, prevalence and mortality are key ideas in NSW Biology Stage 6, Module 8, Non-infectious Disease and Disorders. This topic matters because Module 8 specifically requires students to collect, represent and analyse data on the incidence, prevalence and mortality rates of non-infectious diseases, and to analyse disease patterns in populations using epidemiological studies. HSC materials also regularly test how these measures are calculated and how trends should be interpreted.
In this lesson
the key definitions of incidence, prevalence and mortality
how each measure is calculated
how to tell them apart in exam questions
how to interpret trends in graphs and tables
how these measures are used in epidemiology
Why these measures matter
Epidemiology looks at patterns of disease in populations.
To understand those patterns, biologists use measures such as:
incidence
prevalence
mortality
These measures help answer different questions about disease. One of the most common mistakes is treating them as if they all mean the same thing.
Incidence
Incidence is the number of new cases of a disease in a population over a given time period.
What incidence tells you
Incidence helps show:
how quickly new cases are appearing
whether disease risk is increasing or decreasing
how strongly a disease is spreading or developing in a population
Key idea
Incidence is about new cases, not all existing cases.
Prevalence
Prevalence is the total number or proportion of people in a population who have a disease at a particular time, or over a stated period.
What prevalence tells you
Prevalence helps show:
how widespread a disease is
how much disease is currently present in the population
the overall burden of disease
Key idea
Prevalence includes existing cases, not just new ones.
A 2025 HSC multiple-choice question shows that if a treatment prolongs life but does not cure the disease, prevalence increases while incidence stays the same. That is a very useful HSC-style distinction.
Mortality
Mortality is the number of deaths caused by a disease in a population over a given time period.
What mortality tells you
Mortality helps show:
how deadly a disease is at the population level
whether treatment and management may be improving
whether deaths are rising or falling over time
Key idea
Mortality is about deaths, not just cases.
Key definitions compared
Measure | What it counts | Main question it answers |
Incidence | New cases | How many new cases are appearing? |
Prevalence | All existing cases | How widespread is the disease? |
Mortality | Deaths | How many people are dying from the disease? |
How to calculate incidence
A common formula is:
Incidence rate = (number of new cases ÷ population) × 100,000
Sometimes incidence is also shown as a percentage, depending on the question.
Example
A 2023 HSC question gave 642 new cases of mesothelioma in a population of 26 million and asked for the incidence as a percentage.
Important point
Always check:
whether the question wants a percentage
or a rate per 100,000
or another stated population size
How to calculate prevalence
A common formula is:
Prevalence = (number of existing cases ÷ population) × 100,000
or
Prevalence = (number of existing cases ÷ population) × 100
if the question asks for a percentage.
Important point
Use all current cases, not just new ones.
How to calculate mortality
A common formula is:
Mortality rate = (number of deaths ÷ population) × 100,000
or sometimes as a percentage if the question asks for that.
Example
The Module 8 sample material gives melanoma data using both:
incidence per 100,000
mortality rate in deaths per 100,000
Reading the units carefully
This is one of the biggest exam skills in this topic.
Before interpreting data, check:
the time period
whether values are per 100,000
whether values are percentages
whether the graph is showing incidence, prevalence or mortality
If you ignore the units, it is easy to make the wrong conclusion.
How to interpret trends
To interpret a trend, do more than just say “it goes up” or “it goes down”.
A strong trend answer should
identify whether the value increases, decreases or stays stable
use the correct epidemiological term
include some data if possible
explain why the trend may be happening
Example of incidence trends
The Module 8 sample material shows that melanoma incidence:
increased from 26.6 per 100,000 in 1982
to about 49.5 per 100,000 in 2002
then stayed relatively steady after that
Why this matters
This means you should be able to:
identify the trend
describe it quantitatively
explain why the trend may lag behind a public health campaign
The same material explicitly notes the lag between exposure to a disease-causing factor and later disease appearance.
Interpreting prevalence trends
The Module 8 sample material on smoking shows how prevalence can change over time in a population.
It explains that:
male smoking prevalence steadily decreased
female smoking prevalence rose until 1980, then fell
Why this matters
When answering prevalence questions, it is useful to:
compare groups
describe when the trend changes
include quantitative detail where possible
Interpreting mortality trends
Mortality trends often need a different explanation from incidence trends.
Important idea
A disease can have:
rising incidence
but fairly stable mortality
if:
treatment improves
detection happens earlier
management becomes more effective
The melanoma example in the Module 8 sample material shows this clearly. Incidence rose strongly over time, while mortality stayed relatively steady at around 5 deaths per 100,000.
Common HSC-style interpretation points
Incidence increasing
This suggests more new cases are developing.
Prevalence increasing
This suggests the disease is becoming more widespread, or that people are living longer with it.
Mortality decreasing
This may suggest better treatment, earlier detection, or improved management.
Incidence unchanged but prevalence increasing
This can happen when people live longer with the disease, which is exactly the logic tested in the 2025 HSC multiple-choice question.
How to answer trend questions well
A strong exam response usually does three things:
1. Identify the trend
State what is happening.
2. Use data
Include numbers, dates or comparisons.
3. Explain the trend
Link it to a biological or epidemiological reason.
The Module 8 sample materials repeatedly show that higher-mark answers do more than just identify a trend, they account for it.
Worked example
Exam-style question
Explain the difference between incidence and prevalence.
Worked answer
Incidence is the number of new cases of a disease in a population over a given time period. Prevalence is the total number or proportion of people in the population who currently have the disease. Incidence shows how quickly new cases are appearing, while prevalence shows how widespread the disease is.
Why this works
This answer:
defines both terms clearly
compares them directly
explains what each one measures
Common mistakes
Mixing up incidence and prevalence.
Using new cases when calculating prevalence.
Using all existing cases when calculating incidence.
Forgetting to check whether the answer should be a percentage or per 100,000.
Describing a graph trend without including any data.
Explaining mortality and incidence as if they always change in the same direction.
Quick quiz
What is incidence?
What is prevalence?
What is mortality?
Why might prevalence increase even if incidence stays the same?
Why is it important to check the units on a disease-data graph?
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