Epidemiology Kept Simple

1
Epidemiology Kept Simple

• Chapter 6
• Incidence and Prevalence

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Case Counts

• Prevalence count no. of cases at particular
  time
• Incidence count no. of case onsets that
  accumulate over time

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Inadequacy of Case Counts

• Counts without context seldom useful in epi
• Convert count to ratios
• Numerator case count
• Denominator population size

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Types of Ratios Used in Incidence and Prevalence

• Rates (?)
• Change per unit time
• Can be used to describe incidence
• Proportions (p)
• Mathematically distinct from rates
• Can be used for incidence and prevalence
• Odds (o)
• Rarely used for descriptive purposes but are
  still used in some multivariate models

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Types of Populations

• Closed Cohorts
• Opened Dynamic

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Closed Populations

• No immigration or emigration
• Average age increases over time
• Size decreases over time (with die-off)

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Open Populations

• Dynamics
• Inflow immigration birth
• Outflow emigration death
• Average age may increase, decrease, or stay the
  same
• Size may increase, decrease, or stay the same
• Stable (stationary) population inflow
  balances outflow maintains constant size and
  age structure over time

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6.2 Incidence Proportion

• Synonyms risk, cumulative incidence
• Use in closed population only
• Numerator onsets
• Denominator no. _at_ risk
• excludes those not capable of developing the
  condition

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Incidence Proportion (Illustration)

• Recruit 1000 women aged 60 69
• 100 had hysterectomies before age 60
• 900 at risk of uterine cancer
• Follow for 10 years
• 10 develop uterine cancer
• pincidence 10 women / 900 women .011
• Interpretation
• Average risk
• Estimated probability of developing disease
• e.g., 10-year average risk in group is .0111
  (1.1)

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6.3 Incidence Rate

• Synonyms incidence density, person-time rate
• Can be applied to open and closed populations
• Numerator onsets
• Denominator person-time _at_ risk

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Understanding Person-Time

• One person observed for 1 year 1 person-year
• Two people observed for ½ year each 1
  person-year
• Four people observed for ¼ year each 1
  person-year
• 52 people observed for a week each 1
  person-year
• etc. (ad nauseum)

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Summing Person-Time in a Cohort

• In figure below
• Person 1 has 25 years before death
• Person 2 has 50 years before death
• Person-time in cohort 25 50 75 years
• Rate 2 onsets / 75 years
• .000267 onsets / year

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Interpretation of Incidence Rate

• Speed
• How quickly events develop in population
• Inverse of expected waiting time
• Let ? represent rate and ê represent life
  expectancy
• ê 1 / ?
• e.g., mortality rate .0266667 year-1 (prior
  slide)
• ê 1 / .0266667 year-1 37.5 years (average
  lifespan)
• When disease rare (pincidence ? 5), ?incidence ?
  pincidence
• i.e., rate estimates risk for rare diseases

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Estimating Person-Time in an Open Population

• Person-time ? (average population size) (time
  of observation)
• Example
• 2,391,630 deaths in US in 1999
• Average population size in 1999 272,705,815
• One year observation period

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Other Types of Epi Rates

• The concept of rate can be applied to various
  risk units
• Examples shown below and on pp. 133134

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Prevalence

• Proportion with condition at point or period of
  time
• Example
• Recruit 1000 women aged 60 69
• 100 had hysterectomies before age 60
• Prevalence of hysterectomy 100 women / 1000
  women .1

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Interpretation of Prevalence

• Prevalence probability person selected at
  random has condition
• c.f. to incidence proportion (probability person
  selected at random will develop condition over
  stated time)

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Dynamics of Prevalence (p. 185)
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Relation Between Incidence and Prevalence Rate

• assumes disease rare steady- state
• e.g., if rate is .1 year-1 and average duration
  is 2 years, prevalence .1 year-1 1 year .2

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Use of Multiplier to Report Incidence and
Prevalence

• To report per m people, multiply by m
• e.g. Report rate of 0.00877 year-1 per 100,000
  (m 100,000)
• 0.00877 year-1 100,000 877 per 100,000
  person-years
• e.g., Report prevalence of .1 per 100 persons
• .1 100 10 per 100 persons
• Reporting per m does not change the value
• But makes it easier for lay public to understand