Epidemiology and screening: incidence, prevalence, standardised mortality and screening biases
Incidence and prevalence, crude and standardised rates, the SMR, the Wilson–Jungner criteria, and the three biases that make screening look better than it is.
Reviewed and kept current
Last editorial review: 28 September 2026 · Next scheduled refresh: March 2027.
Clinical vignette
Begin with the patient. Read the case, weigh the options, then work through the unit.
Mr L, aged 60, asks to be screened with a new test for a cancer. He has read that the test detects the cancer two years earlier than symptoms would. In the screened group, five-year survival from diagnosis improved from 40% to 60%. He has no symptoms. He says: “That must save lives. I want the test, even if I have to pay for it.”
The same report states that the age at death was unchanged.
Mr L expects you to agree. The survival figure and the age at death point in different directions.
Options to weigh
- Agree that the test saves lives on the strength of the improved five-year survival.
- Explain that earlier diagnosis starts the survival clock earlier, so survival from diagnosis can lengthen without delaying death.
- Ask whether a randomised comparison shows a fall in disease-specific mortality.
- Explain that screening may detect disease that would never have caused symptoms or death.
- Check whether the test forms part of a programme recommended by the UK National Screening Committee.
Reflect
How would you explain these figures so that Mr L can make an informed choice about screening, and what would you be careful not to say?
What this unit covers
Pocket-sized and self-contained: three AKT-style questions, the answers explained, an expanded discussion with hyperlinked guidance, a decision algorithm and a certificate.
By the end of this unit you will be able to
Five steps, in order
Answer first, then learn. Each section unlocks as you progress, and your progress is saved in this browser.
1Three single best answer questions
Commit to an answer for all three single best answer questions, then submit. Sections 2–4 open on submission; your certificate opens at 2/3 or better.
Question 1 of 3
A new treatment for a chronic incurable disease significantly prolongs survival but does not cure it or change the rate at which new cases arise.
What will happen to the incidence and prevalence of the disease?
Question 2 of 3
An ICB public-health report shows that the standardised mortality ratio (SMR) for coronary heart disease in one locality is 130, using England as the standard population.
What does this mean?
Question 3 of 3
A new screening test detects a cancer two years earlier than symptomatic presentation. Five-year survival from diagnosis improves from 40% to 60% in the screened group, but the age at death is unchanged.
Which bias best explains the apparent improvement in survival?
2Answers explained
Question 1
Correct answer B. Incidence unchanged; prevalence rises
New cases arise at the same rate (incidence unchanged) but people live longer with the disease (duration ↑), so prevalence rises. This is why the prevalence of HIV, type 1 diabetes and treated heart failure has increased.
Question 2
Correct answer B. The locality had 30% more coronary deaths than expected given the age structure of its population
“Expected” deaths are what the locality would have if its age-specific death rates matched England’s (indirect standardisation). An SMR of 130 means 30% more deaths than expected after allowing for age (and usually sex). An SMR of 100 = as expected; below 100 = fewer.
Question 3
Correct answer A. Lead-time bias
Lead-time bias: diagnosing earlier starts the survival “clock” earlier, so survival from diagnosis lengthens even if the date of death is unchanged. The unbiased measure of a screening programme’s benefit is disease-specific mortality in a randomised comparison, not survival from diagnosis.
- Length-time bias: screening preferentially detects slow-growing (better-prognosis) tumours, which have a long pre-clinical phase.
- Overdiagnosis: detection of disease that would never have caused symptoms or death (e.g. some low-grade prostate cancers, DCIS).
3Expanded discussion, worked examples & guidance
1. Incidence and prevalence
Worked example 1
A practice of 12,000 patients has 600 people with diagnosed type 2 diabetes on 1 January. During the year, 60 new diagnoses are made among the 11,400 without diabetes.
- Point prevalence (1 Jan) = 600/12,000 = 5%.
- Annual incidence = 60/11,400 = 5.3 per 1,000 per year (the denominator is the population at risk).
Answer. Prevalence 5%; incidence 5.3 per 1,000 per year.
2. Crude and standardised rates
Crude rates mislead when populations differ in age: a coastal practice with many older people will have a higher crude death rate. Direct standardisation applies each population’s age-specific rates to a standard population (e.g. the European Standard Population) to give age-standardised rates that can be compared. Indirect standardisation applies standard rates to the local population to calculate expected deaths and the SMR — preferred when local numbers are small.
Try it · live calculator
Standardised mortality ratio
Worked example 2 — calculating an SMR
A PCN recorded 84 deaths from COPD. Applying national age-specific COPD death rates to the PCN population gives 70 expected deaths.
- SMR = 84/70 × 100.
Answer. SMR = 120 — 20% more deaths than expected for the age profile. Check the 95% CI before calling it significant; small numbers give wide intervals.
3. Screening
Screening tests apparently healthy people to identify those at higher risk of a condition, so that earlier treatment improves outcomes. The Wilson and Jungner (WHO, 1968) principles, updated in the UK National Screening Committee (UK NSC) criteria, cover:
- The condition — an important health problem, with a recognisable latent or early symptomatic stage and a well-understood natural history.
- The test — simple, safe, precise, validated and acceptable, with an agreed cut-off.
- The intervention — effective treatment, with better outcomes from early treatment than late.
- The programme — RCT evidence that screening reduces mortality or morbidity; benefit outweighs harm; cost-effective; adequate staffing, quality assurance and informed choice.
UK examples include abdominal aortic aneurysm (AAA) screening for men in their 65th year by a single abdominal ultrasound, bowel cancer screening with FIT, breast, cervical (HPV primary), diabetic eye and antenatal/newborn programmes. Programme details change — check the UK NSC and NHS England pages before an exam.
4. The screening biases
| Bias | Mechanism | Effect |
|---|---|---|
| Lead-time | Earlier diagnosis moves the start of the survival clock | Survival from diagnosis ↑ without delaying death |
| Length-time | Slow-growing disease spends longer in the detectable pre-clinical phase | Screen-detected cases have better prognosis regardless of treatment |
| Overdiagnosis | Detects disease that would never have caused harm | Incidence ↑, survival ↑, harms of treatment, mortality unchanged |
| Healthy-volunteer (selection) | People who accept screening are healthier and more health-conscious | Attenders do better than non-attenders even without benefit from screening |
- Screening ≠ case-finding. Case-finding targets people already in contact with services with risk factors (e.g. the CVD Modern Service Framework’s case-finding for hypertension and AF).
- Only an RCT with mortality as the outcome avoids lead-time and length-time bias.
- Incidence uses the population at risk (exclude existing cases) in the denominator.
References and further reading
Guidance checked at the last editorial review (28 September 2026). Always use the current version.
- UK National Screening Committee — criteria for a screening programme
- Wilson JMG, Jungner G. Principles and practice of screening for disease. WHO, 1968
- NHS population screening programmes
- Office for National Statistics — age-standardised mortality rates
- RCGP GP Curriculum 2025 — Population and planetary health
4Decision algorithm
Should we screen for this condition?
Start
Proposed screening programme for condition X.
Decision
Is X an important problem with a detectable early phase and a known natural history?
Decision
Is there a simple, safe, validated and acceptable test with an agreed cut-off?
Decision
Does early treatment improve outcomes compared with treatment at presentation?
Decision
Is there RCT evidence that screening reduces mortality or morbidity, with benefits outweighing harms (including overdiagnosis)?
Outcome
Is the programme cost-effective, quality-assured, equitable and based on informed choice? → The UK NSC recommends it.
Revision summary; apply clinical judgement and local pathways.
5Certificate of completion
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