a short introduction to epidemiology chapter 2a: conducting a cohort study
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A short introduction to epidemiology Chapter 2a: Conducting a cohort study. Neil Pearce Centre for Public Health Research Massey University Wellington, New Zealand. Chapter 2 (additional material) Cohort studies. This presentation includes additional material on conducting a cohort study - PowerPoint PPT PresentationTRANSCRIPT
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A short introduction to epidemiology
Chapter 2a: Conducting a cohort
study
Neil PearceCentre for Public Health
ResearchMassey University
Wellington, New Zealand
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Chapter 2 (additional material)
Cohort studies
• This presentation includes additional material on conducting a cohort study
• It particularly focuses on occupational studies, because these often have good historical exposure data
• Brief mention is also made of other types of cohort studies
• More information on data analysis is given in chapter 9
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Chapter 2 (additional material)
Cohort studies
• Defining the cohort
• Defining exposure
• Follow-up
• Data analysis
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Birth End of Follow up
Death
other death
lost to follow up
“non-diseased”
symptoms
severe disease
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A Hypothetical Cohort Study
Exposed Non-exposed Ratio
Cases 1,813 952
Non-cases 8,187 9,048
Total 10,000 10,000
Person-years 90,635 95,163
Incidence rate 0.0200 0.0100 2.00
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Defining the Cohort
• Cohort studies are most frequently conducted in two different contexts:
• Studies based on a particular community (e.g. Framingham, birth cohort studies)
• Studies based on a particular occupational group (e.g. lung cancer in asbestos workers)
• Each type of study may involve an external comparison (e.g. with national mortality rates) or an internal comparison
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Defining the Cohort
• What is the appropriate comparison population for an external comparison?
• What is the most appropriate geographical area? (national, regional, local)
• What is the most appropriate population within this area? (all persons, all employed persons, same social class)
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The Healthy Worker Effect
William Ogle (1885)
“Some occupations may repel, while others encourage the unfit at the age of starting work.”
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Employed Non-employed
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The Healthy Worker Effect
• Healthy individuals are more likely to obtain employment
• Unhealthy individuals are more likely to leave employment
• The health worker effect is particularly strong for heart disease and non-malignant respiratory disease, and is generally weaker for cancer
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The Healthy Worker Effect
Checkoway et al (1985). Mortality in energy research laboratory workers
Cause Observed Expected SMRCancer 194 250.0 0.78Heart disease 344 459.9 0.75Respiratory 42 69.2 0.61Other 386 540.9 0.71Total 966 1320.0 0.73
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Defining an Occupational Cohort
• All workers ever employed in one factory
• Workers from multiple plants, engaged in the same industrial process
• Members of a trade union or professional organisation
• Registered cases of occupational disease (e.g. asbestosis)
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Defining an Occupational Cohort
Cohort enumeration
• Plant personnel records
• Union membership listings
• Other data sources
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Example of an Employment Record
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Defining an Occupational Cohort
Cohort restriction
• All employees
• Workers first employed on or after a particular date
• Gender, ethnicity
• Workers with a minimum employment duration (e.g. one month)
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Defining the Cohort
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Chapter 2 (additional material)
Cohort studies
• Defining the cohort
• Defining exposure
• Follow-up
• Data analysis
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Exposure and dose
Exposure: the presence of a substance in the environment external to the worker (external/environmental)
Dose: The amount of a substance that reaches susceptible targets in the body (internal)
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Measures of exposure
Intensity of exposure
Duration of exposure
Cumulative exposure
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Types of exposure data used in occupational studies
• Quantified personal measurements
• Quantified area- or job-specific data
• Ordinally ranked jobs or tasks
• Duration of employment in the industry
• Ever employed in the industry
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Chapter 2 (additional material)
Cohort studies
• Defining the cohort
• Defining exposure
• Follow-up
• Data analysis
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Start of Follow-up
Follow-up starts on the first date that each worker satisfies the eligibility criteria for the cohort, I.e. the most recent date of:
• Start of employment (plus minimum employment period, e.g. one month)
• Date of start of study
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Vital StatusDeaths ( or e.g. cancer registrations) can be identified through national recordsFor non-deceased participants, vital status can be identified through sources such as:
• Employment records• Superannuation records• Electoral rolls• Drivers license records• Postal questionnaire
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End of Follow-up
Follow-up ends on the last date that each worker satisfies the eligibility criteria for the cohort, I.e. the earliest date of :
• Date of death
• Date of emigration
• Last date known to be alive
• Date of finish of study
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Follow-Up
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Year 60 61 62 63 64 65 66 67
Age 27 28 29 30 31 32 33 34 (at risk)
Employment 1 1 1 1 1 0 0 0 status
Follow-up 0 1 2 3 4 5 6 7
Employment History of a Worker
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Accumulation of Person-Years
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Chapter 2 (additional material)
Cohort studies
• Defining the cohort
• Defining exposure
• Follow-up
• Data analysis
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Data Analysis
• Each worker accumulates person-years of follow-up in categories of age, calendar period (and gender)
• These are used to calculate the expected numbers of deaths from all causes and from specific causes
• These are then compared to the observed deaths in the cohort
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Data Analysis
Age-group Deaths Person-years
40-49 6 1200
50-59 27 2340
60-69 98 3750
70-79 48 975
Total 179 8265
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Data Analysis
• We wish to compare the death rates in this cohort with those in some standard external comparison population (usually the national population)
• This involves calculating the expected number of deaths in the cohort if it had had the same death rate as the comparison population
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Data Analysis
Age-group
Deaths
Person-years
National rates Expected deaths
Ratio 40-49 6 1200 2.5/1000 3.00 2.00
50-59 27 2340 6.1/1000 14.27 1.89
60-69 98 3750 12.4/1000 46.50 2.11
70-79 48 975 25.0/1000 24.38 1.97
Total 179 8265 88.15 2.03
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Strategies of Data Analysis
• Overall cohort analysis– assumes that all members of
cohort are exposed
– cohort can be stratified by duration of employment
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Strategies of Data Analysis
• Subcohort analysis– job categories
– duration of employment within job categories
– ordinally ranked job categories
– cumulative exposure
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Ever Employed in the Industry:SMRs Amongst While Male Phosphate Industry Workers 1949-78 (Checkoway Et Al, 1985)
Causeof death Observed Expected SMR
All causes 1,620 1,623.8 1.00
Lung cancer 117 95.9 1.22
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Duration of Employment:Lung Cancer Amongst White Male Phosphate Workers 1949-78 (Checkoway Et Al, 1985)
Years of work Lung Cancers SMR1-4 29 1.365-9 17 1.18
10-19 29 1.0920-29 25 1.0530+ 17 1.62
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Unranked Jobs:Respiratory Disease Mortality in Metal Trades Occupations (Beaumont and Weiss, 1980)
Job category Lung Cancers SMR
Welders 53 1.31
Ship Fitters 12 0.57
Burners 11 1.57
Boilermakers 14 1.98
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Ordinally Ranked Jobs:Lung Cancer Amongst Amosite Asbestos Factory Workers (Acheson Et Al, 1984)
Exposure Lung Cancers SMR
Background 11 1.06
Low 12 1.34
Medium 41 2.25
Heavy 8 4.25
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Cumulative Exposure:Lung Cancer Amongst Chrysotile Asbestos Textile Workers (Dement Et Al, 1983)
Cumulative exposure Lung SMR(fibers/cc x days) Cancers
<1,000 5 1.33
1,000-9,999 10 2.69
10,000-39,999 7 3.20
40,000-99,000 11 10.00
100,000+ 2 14.93
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A short introduction to epidemiology
Chapter 2a: Conducting a cohort
study
Neil PearceCentre for Public Health
ResearchMassey University
Wellington, New Zealand