Revision notes · Lecture 4 · Free
Measures of disease frequency
In one line: count the new cases and you have incidence (risk). Count all the cases and you have prevalence (burden).
What this lecture's questions test
Out of the 30 questions on this lecture:
| Idea | Questions |
|---|---|
| Incidence or prevalence: which one to use | 7 |
| Two kinds of incidence | 5 |
| Prevalence = incidence × duration | 6 |
| Case fatality or mortality | 5 |
| Calculations | 7 |
1. Incidence or prevalence?
| If the question is about… | Use |
|---|---|
| New cases, risk of getting it, cause, a prevention programme | Incidence |
| Existing cases, burden, planning services, "at a point in time" | Prevalence |
- Gradual onset (diabetes, hypertension): the start date is unknowable, so use prevalence.
- Recurrent episodes (colds, diarrhoea, asthma attacks): count episodes, so use incidence.
- A lasting condition (a diabetic foot ulcer): use prevalence.
- Birth rate is not a measure of disease frequency.
Remember it: incidence is the tap (new water coming in). Prevalence is the water in the tub.
2. Prevalence = incidence × duration
The tub empties through two drains: cure and death.
| Prevalence goes up with | Prevalence goes down with |
|---|---|
| Longer duration | Shorter duration |
| Low cure rate | High cure rate |
| Low case fatality (people survive longer) | High case fatality |
| Patients moving in | Patients moving out |
| Better case detection |
- Chronic disease → high prevalence, low incidence.
- Acute disease → high incidence, low prevalence.
Trap: "low case fatality" sounds like good news, but it raises prevalence, because more people stay ill for longer.
3. The two kinds of incidence
| Cumulative incidence | Incidence density | |
|---|---|---|
| Also called | Risk, incidence proportion, attack rate | Incidence rate |
| Numerator (top) | New cases | New cases (or episodes) |
| Denominator (bottom) | People at risk at the start | Person-time at risk |
| A true rate? | No, a proportion | Yes |
- Leave out people who already have the disease, from the numerator and the denominator. They are not at risk.
- Person-time: add up how long each person was watched, stopping when they get the disease or leave the study.
- Prevalence never uses person-time. Only incidence density does.
4. Case fatality or mortality?
| Calculation | Tells you | |
|---|---|---|
| Case fatality ratio | Deaths ÷ cases | Severity: how deadly it is for those who get it |
| Disease-specific mortality | Deaths ÷ whole population | Impact on the population |
- Case fatality is not a true rate: there is no time in its denominator.
Remember it: look at the denominator. Cases → case fatality. Everyone → mortality.
5. The calculations
"Per 1000" means × 1000. 1,00,000 = one lakh = 100,000.
- Prevalence: 30 cases in 6,000 people → 30 ÷ 6,000 × 1000 = 5 per 1000.
- Case fatality: 4 deaths among 80 cases → 4 ÷ 80 = 5%. Ignore distractors such as "how many were admitted".
- Incidence density: 600 episodes in 200 children watched for a year → 600 ÷ 200 = 3 episodes per child-year.
- Person-years: 50 people for a year, 5 get the disease, then 45 people for a year → 50 + 45 = 95 person-years.
- Incidence with old cases present: 200 people, 20 already ill, 18 new cases → 18 ÷ (200 − 20) = 10%.
Last look before the exam
- New cases → incidence. All cases → prevalence.
- Is a prevention programme working? → incidence.
- Prevalence = incidence × duration. Low case fatality raises it.
- Person-time → incidence density only.
- Case fatality = severity (÷ cases). Mortality = impact (÷ population).