changed 5 years ago  
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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
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