Incidence and standardised rates
- Incidence is the rate of growth of new cases
- In susceptible people
- Measured in terms of new cases per unit of susceptible population
- Reported as incidence density or cumulative incidence
Concept of person time
- Number of persons * Time period
- Interchangeable
- 1 person x followed for 1 year = 1 person-year
- 10 people x followed 1 year = 10 person-year
- 1 person x followed 10 years = 10 person-year
Example Incidence table of diabetes in a population
| Year | Population | New cases | Incidence |
|---|---|---|---|
| 1 | 1000 | 20 | 20.00 |
| 2 | 980 | 30 | 30.61 |
| 3 | 950 | 40 | 42.10 |
| 4 | 910 | 50 | 54.94 |
| 5 | 860 | 40 | 46.51 |
Incidence can show the effects
- Change with intervention
- Change with change of exposure
Change of incidence rates
Prevalence in two populations
What to interpret?
- Different age structures
- How do we make sense?
How to use a third population
- World standard population
- SEGI population
SEGI population
| Age in years | Population |
|---|---|
| <5 | 24000 |
| 5-24 | 72000 |
| 25-39 | 40000 |
| 40-59 | 42000 |
| 60-79 | 20000 |
| 80+ | 2000 |
Application of standardisation
| Age | Count (A) | Count (B) |
|---|---|---|
| <5 | 24 | 240 |
| 5-24 | 720 | 720 |
| 25-39 | 800 | 800 |
| 40-59 | 1470 | 1890 |
| 60-79 | 800 | 400 |
| 80+ | 30 | 30 |
Age-standardised cancer in NZ
Summary
- Prevalence and incidence are descriptive measures
- Prevalence is a static measure
- Incidence is a rate
- Incidence helps to understand the trend
- Age-standardisation helps to adjust for different populations
Incidence and standardised rates Incidence is the rate of growth of new cases In susceptible people Measured in terms of new cases per unit of susceptible population Reported as incidence density or cumulative incidence
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