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Cohort study

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Cohort study Retrospective cohort studies Retrospective cohort studies * * * * * * * * * * * * * * * * * * * * Cohort Study Design Type of Cohorts Open cohort A group ... – PowerPoint PPT presentation

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Title: Cohort study


1
Cohort study
2
What is a cohort?
  • One of 10 divisions of a Roman legion
  • Group of individuals
  • sharing same experience
  • followed up for specified period of time
  • Examples
  • birth cohort
  • cohort of guests
  • occupational cohort of chemical plant workers

3
follow-up period
4
  • Calculate measure of frequency
  • Cumulative incidence
  • Incidence proportion
  • Attack rate (outbreak)
  • Incidence density

end of follow-up
5
Cohort studies
  • Purpose
  • Study if an exposure is associated with
    outcome(s)?
  • Estimate risk of outcome in exposed and
    unexposed cohort
  • Compare risk of outcome in two cohorts
  • Cohort membership
  • Being at risk of outcome(s) studied
  • Being alive and free of outcome at start of
    follow-up

6
Cohort studies
7
Cohort studies
exposed
unexposed
8
Prospective cohort study
Disease occurrence
Exposure
time
9
Retrospective cohort studies
10
Disease occurrence
Exposure
time
Case study Salmonella in Belfast
11
Cohort Study DesignDirectionality
12
(No Transcript)
13
Cohort studies
14
Cohort Study Design
T
PAR Population at Risk S Sampling
design T Elapsed time
15
Recipe Cohort study
  • Identify group of
  • exposed subjects
  • unexposed subjects
  • Follow up for disease occurrence
  • Measure incidence of disease
  • Compare incidence between exposed and unexposed
    group

16
Study Population
  • Define Population at Risk using inclusion
    criteria
  • Individuals with outcome of interest at time of
    screening and enrollment are not eligible for
    study
  • Sub-clinical presentation of diseases may be
    present challenges in defining the cohort

17
  • Our objective is to compare
  • An incidence rate in an exposed population
  • to the rate that would have been observed
  • in the same population, at the same time
  • if it had not been exposed

18
Prospective Cohort Study
  • Measures of association
  • Relative risk (ratio of proportions)
  • Odds ratio

19
Study Design Prospective Cohort Study
  • Relation between oral contraceptive use and
    circulatory disease
  • Study design
  • Identify 23,000 users and 23,000 nonusers of oral
    contraceptives
  • Follow and ascertain presence or absence of
    circulatory disease

20
Cohort Study Design
  • An epidemiologic design in which the incidence of
    a disease (or condition) is estimated and
    compared among exposed and unexposed individuals.

21
Cohort Study DesignRationale
  • Cohort study designs evolved because of the need
    for information on the length of survival and the
    natural history of disease
  • Clinical and public health interest

22
Cohort Study DesignHistory
  • Prospective cohort studies
  • Chronic Disease Cohorts (20th Century)
  • Framingham study of cardiovascular disease, 1948
  • Japanese atomic bomb survivors, 1946
  • British physician study, 1950s
  • Colorado Plateau uranium miners, 1950s
  • Retrospective cohort studies
  • Aniline-dye occupational cohort, 1954

23
Framingham StudyCohort Assembly
24
Measuring Exposure
  • Measuring exposure is one of the fundamental
    activities of a cohort study
  • Exposure measurement must be comparable for all
    members of the cohort
  • Carefully defined in advance of study
  • Specific attention should be given to the
    accuracy and precision of proposed measurements
  • Pilot studies often needed

25
Determining the Exposure
  • Valid means of determining exposure include
  • Questionnaires (e.g., age, smoking history)
  • Laboratory tests (e.g., cholesterol, hemoglobin)
  • Physical measurements (e.g., blood pressure,
    height)
  • Special procedures (e.g., electrocardiogram,
  • x-rays)
  • Medical records

26
COHORT STUDIES
  • Cohort Study
  • Key Point
  • Presence or absence of risk factor is determined
    before outcome occurs.

27
Comparison (Control) Groups for Cohort Studies
  • Internal controls
  • With a one-sample (population-based) cohort,
    exposure is unknown until after the first period
    of observation
  • Example
  • Select the cohort (such as all residents of a
    given neighborhood)
  • All members of the cohort are then given first
    round questionnaires, and/or clinical
    examinations, and/or testing to determine
    exposure
  • The cohort is then divided into exposure
    categories based on those results

28
Comparison (Control) Groups for Cohort Studies
(cont.)
  • External controls
  • If everyone in a cohort is exposed (such as
    workers in an industry), a separate cohort as
    similar as possible to the exposed in terms of
    income, education, geography, and age should be
    sought
  • Example
  • Workers in a neighboring but unexposed industry

29
Comparison (Control) Groups for Cohort Studies
(cont.)
  • Known population rates
  • If a comparison group cannot be assembled, known
    population rates of outcomes may be acceptable
    under some circumstances, if they are adjusted
    for the variables of interest
  • For lung cancer, however, rates are based on the
    population and are not adjusted for smoking
  • They are not, therefore, instructive to compare
    to populations with high smoking rates, such as
    miners

30
Outcome Definition
  • Primary outcome - the main event that will be
    related to the exposure
  • Failure-time outcomes
  • Death
  • Disease occurrence
  • Repeated measures
  • Secondary outcomes - other events that are of
    interest and may corroborate the findings of the
    main outcome

31
Follow-up
  • Completeness and non-participation
  • 90 rule of thumb
  • All subjects must have an equal likelihood for
    detecting the outcome
  • Disease ascertainment must be comparable between
    the exposed and unexposed subjects
  • Number of visits
  • Reasons for additional evaluations
  • Follow-up mechanisms
  • Active
  • Passive

32
Follow-up
  • Passive Surveillance
  • Hospitals
  • Disease Registries
  • Clinics or physician offices
  • Surveillance systems, e.g., National Death Index,
    CDC reportable conditions
  • Active surveillance
  • Systematic evaluations for outcome of interest
  • Regular time intervals
  • In all study subjects

33
Cohort Study DesignTypes of Cohorts
  • Fixed Cohort
  • A group of individuals recruited and enrolled at
    a uniform point in the natural history of a
    disease or by some defining event
  • Cohort does not take on new members after it is
    assembled
  • Examples
  • Patients admitted to the ER with acute MI
  • Survivors of Hiroshima bombings
  • Children born to HIV-infected mothers

34
Cohort Study DesignType of Cohorts
  • Open cohort
  • A group of individuals recruited and enrolled
    through a mechanism that allows for in and out
    migration of people
  • Defined by characteristic other than disease,
    e.g., geographic location, administrative unit
  • Dynamic population
  • Examples
  • Framingham Study

35
Cohort studies
  • Fixed Cohort

X outcome
x
()
x
Exposure
(-)
x
36
Cohort studies
X outcome
  • Dynamic

X
X
()
X
Exposure
X
(-)
Years
37
Cohort studies
38
Cohort Studies
  • Also called follow-up studies, incidence studies,
    panel studies, or prospective studies
  • Begin with a group of individuals free of the
    disease(s) of interest
  • Determine the incidence rate (or mortality rate)
    among the exposed and the unexposed
  • In a prospective cohort study, you collect
    exposure information at the time the study begins
    and follow the cohort for disease status that may
    occur in the future
  • In a retrospective cohort study, you collect
    exposure information from some time in the past
    and construct disease incidence (or mortality)
    from then until the present
  • You may also begin with a retrospective cohort
    and follow it prospectively (sometimes referred
    to as an ambicohort study)

39
Advantages and disadvantages
40
Cohort Study DesignPotential Biases
  • Nonparticipation (selection bias)
  • Selective participation of persons
  • loss to follow-up
  • Information bias
  • The quality of information is different between
    exposed and unexposed subjects
  • Detection bias
  • Subjects with exposure are more (or less) closely
    evaluated for outcome
  • Blinding to exposure status

41
Cohort studies
  • Advantages
  • The cohort study is the gold-standard of
    analytical epidemiology
  • ascertain incidence and natural history
  • investigation of multiple outcomes
  • assessment of many outcomes
  • Useful for rare exposures
  • Temporal relationship between exposure and
    outcome
  • Ascertainment bias minimized
  • Less subject to selection biases
  • outcome not known (prospective)
  • Can directly measure
  • incidence in exposed and unexposed groups
  • true relative risk
  • Can examine multiple effects for a single
    exposure

42
Cohort studies
  • Disadvantages
  • Selection bias
  • Loss to follow up
  • Requires excellent follow-up
  • Large sample size
  • Latency period
  • Time consuming
  • Inefficient for rare diseases
  • Expensive
  • Ethical considerations
  • Exposure can change

43
Effects
44
Cohort studies
  • Rate
  • Rate difference
  • Rate Ratio (strength of association)

Case control studies
  • No calculation of rates
  • Proportion of exposure

45
Measures of Association Prospective Cohort Study
  • Relative risk
  • Also know as risk ratio
  • Ratio of the proportion of cases in exposed group
    compare to proportion in unexposed group

46
Relative Risk
  • General interpretation of relative risk (RR)
  • If RR gt 1 Positive association between disease
    and risk factor
  • 1 No association
  • lt 1 Negative association
  • The reference group is in the denominator
  • Reference group generally chosen as the
    unexposed group

47
Odds Ratios
  • The odds of a disease is defined as
  • or equivalently

48
Odds Ratios
  • For example, if the risk of having disease A were
    .20, the odds of having disease A would be
    .20/.80 .25 (or 1 in 4)
  • Notice, the odds is not the risk but is a
    function of the risk
  • Just as we can compare risk via the risk ratio,
    we can compare odds via the odds ratio
  • Odds ratio is very easy to calculate from a 2 x 2
    table!

49
  • Bottom line
  • Only cohort studies (including clinical trials)
    can yield incidence and relative risk.
  • The odds ratio, (e.g., from a case-control study)
    will always be greater than the relative risk.
    For rare diseases, the odds ratio will be close
    to the relative risk.

50
A cohort study allows to calculate indicators
which have a clear, precise meaning. The
results are immediately understandable.
51
Presentation of cohort data Population at risk
Does HIV infection increase risk of developing TB
among a population of drug users?
Population Cases (f/u 2 years)

HIV 215 8
HIV - 289 1
Source Selwyn et al., New York, 1989
52
Presentation of cohort data Person-years at risk
Tobacco smoking and lung cancer, England Wales,
1951
Person-years Cases
Smoke 102,600 133
Do not smoke 42,800
3
Source Doll Hill
53
Presentation of data Various exposure levels
54
Effect measures in cohort studies
  • Absolute measures
  • Risk difference (RD) Ie - Iue
  • Relative measures
  • Relative risk (RR)
  • Rate ratio
  • Risk ratio

Ie incidence in exposed Iue incidence in
unexposed
55
Does HIV infection increase risk of developing TB
among drug users?
56
Vaccine efficacy (VE)
VE 1 - RR 1 - 0.28 72
57
Cohort study Tobacco smoking and lung cancer,
England Wales, 1951
Source Doll Hill
58
Source population
Pop.
Cases
E
a
P1
I1 a / P1
c
P0
I0 c /P0
E


59
Source population
Pop.
Cases
E
a
P1
I1 a / P1
c
P0
I0 c /P0
E
sample

Cases
Controls
E
a
b
P1 b --- ---- P0 d
c
d
E
60
Source population
Pop.
Cases
E
a
P1
I1 a / P1

c
P0
I0 c /P0
E
a/P1 a . P0 a . d I1 / I0 ------
------- ----- c/P0 c . P1 c . b
a / c ------ b / d
sample

Cases
Controls
E
Since d/b P0 / P1
a
b

P1 b --- ---- P0 d
c
d
E
61
Source population
Pop.
Cases
E
30
100
Case control
Cohort
10
100
E
30/100 30/10 I1 / I0
----------- ------- 3 10/100
20/20
Cases
E
30
20
10
20
E
62
Case control study design
Cases
Controls
Odds ratio
E
a
b
a b a x d ---- --- --- ---- c d b x c
c
d
E
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