## 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