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A short introduction to epidemiology Chapter 2: Incidence studies

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Incidence and Prevalence Incidence is the number of new cases of the condition over a specified period of time ... A short introduction to epidemiology ... – PowerPoint PPT presentation

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Title: A short introduction to epidemiology Chapter 2: Incidence studies


1
A short introduction to epidemiologyChapter 2
Incidence studies
  • Neil Pearce
  • Centre for Public Health Research
  • Massey University
  • Wellington, New Zealand

2
Birth
End of Follow up
Death other death lost to follow up
non-diseased symptoms severe disease
3
Incidence and Prevalence
  • Incidence is the number of new cases of the
    condition over a specified period of time
  • Prevalence is the number of cases of the
    condition at a particular point in time

4
Study Design Options
  • All epidemiological studies are (or should be)
    based on a particular population (the source
    population) followed over a particular period of
    time (the risk period)
  • The different study design options differ only in
    how the source population is defined and how
    information is drawn from this population and
    time period

5
Study Design Options
6
Chapter 2Incidence studies
  • Incidence studies
  • Measures of occurrence
  • Measures of effect
  • Incidence case-control studies
  • Measures of effect

7
Incidence Studies
  • Cohort studies, follow-up studies, longitudinal
    studies, prospective studies
  • Uses all the information on the source population
    and risk period, i.e. collect information on
    exposure and outcome for everyone in the study
  • Compares incidence in exposed and non-exposed

8
Incidence Studies example
  • Taussig et al (1989). Cohort study of 1246
    infants born in Tucson during May 1980-October
    1984
  • Exposures Questionnaire on demographic factors,
    parental history. Cord blood
  • Outcomes Wheezing lower respiratory tract
    illness (3 years), atopy (6 years)

9
A Hypothetical Incidence Study
10
Incidence Proportion (Risk)
  • Proportion of study participants who experience
    the outcome (for the first time)
  • When there are significant losses to follow-up
    the incidence proportion cannot be estimated
    directly

11
A Hypothetical Incidence Study
12
Incidence Rate
  • Number of new cases per unit time (e.g. per
    100,000 person-years)

13
A Hypothetical Incidence Study
14
Incidence Odds
  • Ratio of number of people who experience the
    outcome to the number of people who do not
    experience the outcome

15
Effect Measures in Incidence Studies
  • Rate ratio
  • Risk ratio
  • Odds ratio

16
A Hypothetical Incidence Study
17
Chapter 2Incidence studies
  • Incidence studies
  • Measures of occurrence
  • Measures of effect
  • Incidence case-control studies
  • Measures of effect

18
Birth
End of Follow up
Death other death lost to follow up
non-diseased symptoms severe disease
19
A Hypothetical Incidence Study
20
Odds Ratio
  • OR(1813/8187)/(952/9048) 2.11
  • i.e. odds ratio odds of outcome in the exposed
    divided by odds of outcome in the non-exposed
  • OR(1813/952)/(8187/9048) 2.11
  • i.e. odds ratio odds of exposure amongst the
    cases divided by odds of exposure in the non-cases

21
Odds Ratio
  • We can therefore estimate the odds ratio by
    taking all of the cases and a control sample of
    the non-cases

22
A Hypothetical Case-Control Study
23
Odds Ratio
  • OR(1813/1313)/(952/1452) 2.11
  • This incidence case-control study yields the same
    estimate as would have been obtained by an
    incidence study but with a much smaller number of
    participants because we include all of the cases
    but only a sample of the non-cases

24
Birth
End of Follow up
Death other death lost to follow up
non-diseased symptoms severe disease
25
Methods of Sampling Controls
  • From survivors (non-cases at end of follow-up)
    cumulative sampling
  • From source population case-base sampling
  • From person-years density sampling

26
Methods of Sampling Controls
27
Hypothetical Case-Control Studies
28
Incidence Case-Control Studies example
  • Infante-Rivard (1993), incidence case-control
    study of 3-4 year old children
  • Cases 457 children with first-time diagnosis of
    asthma
  • Controls sample of children of same age without
    a diagnosis of asthma
  • Exposures indoor environmental risk factors

29
Case-Control Studies
  • Retrospective studies
  • Case-referent studies
  • Case-compeer studies

30
Misconceptions About Case-Control Studies
  • Proceeds from effect (disease) to cause
    (exposure), i.e. reverse causality
  • Inherently more prone to bias than cohort studies
  • Odds ratio only approximately estimates the
    relative risk
  • Depends on a rare disease assumption

31
A short introduction to epidemiologyChapter 2
Incidence studies
  • Neil Pearce
  • Centre for Public Health Research
  • Massey University
  • Wellington, New Zealand
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