PH 241: Chapter 2

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PH 241 Chapter 2 Nicholas P. Jewell University
of California Berkeley January 20, 2006
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How do we Measure the Binary Outcome for Disease
Occurrence?

• Incidence/prevalence
• Role of time
• Chronological time
• Exposure time
• age
• Number of contacts
• Incidence (time interval)
• Prevalence (time point or interval)
• Fractions Incidence Proportion
• unitless

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Incidence Proportion

• Definition (D, 1, yes)
• Define risk interval explicitly including time
  scale (calendar year 2005, year of age 55, first
  year after menopause, etc)
• Be at risk at the beginning of the interval
  (define explicitly what at risk means)
• Become an incident case during interval
• Incidence proportion is fraction of at risk
  population who are D
• Cumulative measure

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Incidence Rate

• Introduces time at risk into our thinking
  Incidence Rate (time interval)
• D/cum. time at risk
• Units are now time-1
• Still measure applies to whole interval (so still
  cumulative in that sense)
• Instantaneous Incidence rate Hazard Function
• I(t) is the Incidence Proportion over the time
  interval 0,t

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Hazard Function for Caucasian Males in California
in 1980
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Survival Function (1-I(t)) for Caucasian Males in
California in 1980