A short introduction to epidemiology

Chapter 9: Cohort studies

Neil PearceCentre for Public Health Research

Massey UniversityWellington, New Zealand

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

Chapter 2 (additional material)

Cohort studies

• Defining the cohort• Defining exposure• Follow-up• Data analysis

Birth End of Follow up

Death

other death

lost to follow up

“non-diseased”

symptoms

severe disease

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

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

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 populationwithin this area? (all persons, all employed persons, same social class)

The Healthy Worker Effect

William Ogle (1885)

“Some occupations may repel, while others encourage the unfit at the age of starting work.”

Employed Non-employed

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

The Healthy Worker EffectCheckoway 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

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)

Defining an Occupational Cohort

Cohort enumeration• Plant personnel records• Union membership listings• Other data sources

Example of an Employment Record

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)

Defining the Cohort

Chapter 2 (additional material)

Cohort studies

• Defining the cohort• Defining exposure• Follow-up• Data analysis

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)

Measures of exposure

Intensity of exposure

Duration of exposure

Cumulative exposure

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

Chapter 2 (additional material)

Cohort studies

• Defining the cohort• Defining exposure• Follow-up• Data analysis

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

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

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

Follow-Up

Employment History of a Worker

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 0status

Follow-up 0 1 2 3 4 5 6 7

Accumulation of Person-Years

Chapter 2 (additional material)

Cohort studies

• Defining the cohort• Defining exposure• Follow-up• Data analysis

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

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

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

Data AnalysisAge-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

Strategies of Data Analysis

• Overall cohort analysis– assumes that all members of

cohort are exposed– cohort can be stratified by duration

of employment

Strategies of Data Analysis

• Subcohort analysis– job categories– duration of employment within job categories– ordinally ranked job categories– cumulative exposure

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 1.00Lung cancer 117 95.9 1.22

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

Unranked Jobs:Respiratory Disease Mortality in Metal Trades Occupations (Beaumont and Weiss, 1980)

Job category Lung Cancers SMRWelders 53 1.31Ship Fitters 12 0.57Burners 11 1.57Boilermakers 14 1.98

Ordinally Ranked Jobs:Lung Cancer Amongst Amosite Asbestos Factory Workers (Acheson Et Al, 1984)

Exposure Lung Cancers SMRBackground 11 1.06Low 12 1.34Medium 41 2.25Heavy 8 4.25

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 1.331,000-9,999 10 2.6910,000-39,999 7 3.2040,000-99,000 11 10.00100,000+ 2 14.93

A short introduction to epidemiology

Chapter 2a: Conducting a cohort study

Neil PearceCentre for Public Health Research

Massey UniversityWellington, New Zealand