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COHORT STUDIES
Submitted by,
Reshma S Suresh
OUTLINE
• Overview
• Definitions
• Study design
• Basic measures
• Advantages and disadvantages
• When to apply a cohort design
• Practical considerations
24-10-2023
2
OVERVIEW
Two major categories of Epidemiological studies:
1. Observational studies:
• Cohort studies
• Case-control studies
• Cross-sectional study
Have no control over exposures, simply observe what happens to
groups of people. Examine associations between risk factors and
outcomes.
2. Experimental studies:
• Randomized controlled trials (RCT)
• Non-randomized trial
Explore the association between interventions and outcomes
24-10-2023
3
DEFINITIONS
Cohort
A group of individuals who have characteristics in common
• Ancient Roman military unit, A band of warriors.
• Persons banded together
• Group of persons with a common statistical characteristic [Latin]
• E.g. age, birth date
24-10-2023
4
Examples of cohorts:
• Birth cohort: All individuals in a certain geographic area
born within a given period of time (usually a year)
• Marriage cohort: All persons married within a given period
of time
• Exposure cohort: Individuals assembled as a group based
on some common exposure (e.g. radiation exposure
during nuclear testing, smoking exposure etc)
24-10-2023
5
Cohort Study
• A study in which two or more groups of individuals those are free
of disease and those differ according to the extent of exposure to
a factor of interest, are followed over a period of time to see how
their exposures affect their outcomes
• Presence or absence of risk factor is determined before the
outcome occurs
• A major limitation of cross-sectional surveys and case-control
studies is difficulty in determining if exposure or risk factor
preceded the disease or outcome 24-10-2023
6
INDICATIONS OF A COHORT STUDY
• When there is good evidence of exposure and disease.
• When exposure is rare but incidence of disease is higher among
exposed
• When follow-up is easy, cohort is stable
• When ample funds are available
24-10-2023
7
FRAMEWORK OF COHORT STUDY DESIGN
24-10-2023
8
Population:
Pregnant
women
Exposure:
Smokers
Exposure:
Non-
Smokers
<2.5Kg (LBW)
≥2.5Kg
<2.5Kg (LBW)
≥2.5Kg 24-10-2023
9
Consideration during selection of cohort
• The cohort must be free from disease under study
• Insofar as the knowledge permits, both the groups should be
equally susceptible to disease under study
• Both the groups must be comparable in respect of all variable
which influence the occurrence of disease
• Diagnostic and eligibility criteria of the disease must be defined
beforehand
24-10-2023
10
TYPES OF COHORT STUDIES
1. Prospective cohort studies
2. Retrospective cohort studies
3. Ambi-directional cohort study: elements of prospective and
retrospective cohort are combined. The cohort is identified from past
records and assesses of date for the outcome. The same cohort is
followed up prospectively into future for the further assessment of
outcome.
Both start by identifying subjects based upon the presence or absence of
the exposure of interest, without knowing the outcome at the time their
exposure status is defined
24-10-2023
11
PROSPECTIVE COHORT STUDIES
• The two groups of cohorts (exposed and un-exposed) are followed
prospectively over time to track the development of new disease
• Cohort characterized by determination of exposure levels
(exposed vs. not exposed) at baseline (present) and followed for
occurrence of disease in future
• Here, groups move through time as they age
24-10-2023
12
24-10-2023
13
EXAMPLES
Ramchand R, Ialongo NS, Chilcoat HD. The effect of working for pay
on adolescent tobacco use. Am J Public Health. 2007
Nov;97(11):2056-62. doi: 10.2105/AJPH.2006.094045. Epub 2007
Sep 27. PMID: 17901448; PMCID: PMC2040355
• Cohort: High school students from Baltimore, Maryland
• Exposure: Working for pay
• Outcome: Initiation of tobacco use
• Results: Adolescents who work for pay have a higher risk of initiating
tobacco use
24-10-2023
14
Seang-Mei Saw, Anoop Shankar, Say-Beng Tan, Hugh Taylor, Donald T. H. Tan,
Richard A. Stone, Tien-Yin Wong; A Cohort Study of Incident Myopia in
Singaporean Children. Invest. Ophthalmol. Vis. Sci. 2006;47(5):1839-1844.
doi: https://doi.org/10.1167/iovs.05-1081
• Cohort: Two schools located in the northeastern and southeastern
parts of Singapore, and in 2001 in one school located in the west.
• Exposure: Cycloplegic refraction
• Outcome: Incident myopia
• Results: Parental myopia and children’s IQ are important risk factors
for incident myopia in young Singaporean children
24-10-2023
15
RETROSPECTIVE COHORT STUDIES
• Makes use of historical data to determine exposure level at some
baseline in the past and then determine subsequent disease status
in the present
• Both exposure and disease have occurred at the start of study.
• Data already collected for other purposes.
• The cohort is followed up retrospectively.
• The study period may be many years but the time to complete the
study is only as long as it takes to collate and analyze the data
24-10-2023
16
24-10-2023
17
EXAMPLE
Klungel OH, Heckbert SR, de Boer A, Leufkens HG, Sullivan SD,
Fishman PA, Veenstra DL, Psaty BM. Lipid-lowering drug use and
cardiovascular events after myocardial infarction. Ann
Pharmacother. 2002 May;36(5):751-7. doi: 10.1345/aph.1A308.
PMID: 11978147
• Begin study in 2000 using data already collected via health
plan.
• Cohort: Surviving myocardial infarction (MI) 1986-1996
• Exposed: Lipid lowering therapy use
• Outcome: Cardiovascular events during 6 months following MI
24-10-2023
18
BASIC MEASURES
Measures of disease occurrence:
• Cumulative Incidence
• Incidence Rate (IR)
Measures of association between a factor (exposure) and a
disease:
• Relative Risk (RR)
• Attributable Risk (AR)
24-10-2023
19
1. Cumulative Incidence:
• Risk of developing disease
• New cases of disease/persons at risk (during the same time
period)
– Risk of disease in exposed: a/a+b
– Risk of disease in non-exposed: c/c+d
Disease Non-disease
Exposed a b a+b
Non-exposed c d c+d
a+c b+d
24-10-2023
20
2. Incidence Rate (IR)
• Risk per unit of time
• new cases of disease/Persons at risk*Duration
• Example: IR, Person-time calculation, a 9-year follow-up study
24-10-2023
21
3. Relative Risk (RR):
• Incidence of disease in exposed compared to the incidence
of disease in unexposed
• RR= (a/a+b)/(c/c+d)
24-10-2023
22
Disease Non-disease
Exposed a b a+b
Non-exposed c d c+d
a+c b+d
• Determine the strength of the association between exposure and
disease
• RR=1 (no association)
• RR>1 (exposure increases risk for disease, e.g. RR=2.0 can be
interpreted as two fold increase in risk)
• RR<1 (exposure decreases risk for disease, e.g. RR=0.7 can be
interpreted as 30% decrease in risk)
24-10-2023
23
Example:
(Tuberculosis treatment and breast cancer study)
• Exposed=41/28011=1.5/1000 woman-years
• Non-exposed=15/19025=0.8/1000 woman-years
• RR=Exposed/Non-exposed=1.9
• Results: Women exposed to fluoroscopies had 1.9 times the risk of
breast cancer compared to unexposed women
24-10-2023
24
Attributable Risk (AR):
• The excess risk of disease observed among exposed subjects.
• AR=Exposed - Non-exposed
Example:
(Tuberculosis treatment and breast cancer study)
• Exposed=1.5/1000 woman-years
• Non-exposed=0.8/1000 woman-years
• AR=Exposed - Non-exposed=1.5-0.8=0.7/1000w/y
• Excess IR of breast cancer among women exposed to fluoroscopies was 0.7/1000
woman-year
24-10-2023
25
Which clinical questions does this study
design best answer?
24-10-2023
26
ADVANTAGES
• Gold standard for studying the association between risk factor and
outcome
• Useful for looking at multiple exposures and their interactions
• Can evaluate multiple outcomes/diseases
• Clear time sequence (temporal relationship between exposure and
outcome) strengthens the inference about cause
• Less bias due to prospective evaluation of exposures
• Efficient for rare exposures
• The best or only ethical way, sometimes, to do the study
(situations where randomization is not possible)
24-10-2023
27
DISADVANTAGES
• Time consuming
• The problem of attrition: loss of subjects (e.g. migration or
death from other causes)
• Unexpected changes over time:
–Changes to the environment can influence the association
of disease and possible cause
–Changes in diagnostic criteria and methods
–Changes of staff
• Financial problems: lack of funding and the high costs of
record keeping 24-10-2023
28
OTHER EXAMPLES
24-10-2023
29
• 9600 subjects (rural-to-urban ratio, 4800:4800)
• 40 years of age or older and was carried out from 2001 through
2004.
• From the cohort, 7774 subjects (rural-to-urban ratio, 3924:3850)
participated in the study.
• Main outcome measures were Six-year incidence of POAG and its
associated risk factors.
• In 6 years, incident POAG developed in 129 subjects (2.9%; 95%
confidence interval [CI], 2.4e3.4; male-to-female ratio, 65:64).
Baseline age was a risk factor.
24-10-2023
30
24-10-2023
31

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COHORT STUDIES.pptx

  • 2. OUTLINE • Overview • Definitions • Study design • Basic measures • Advantages and disadvantages • When to apply a cohort design • Practical considerations 24-10-2023 2
  • 3. OVERVIEW Two major categories of Epidemiological studies: 1. Observational studies: • Cohort studies • Case-control studies • Cross-sectional study Have no control over exposures, simply observe what happens to groups of people. Examine associations between risk factors and outcomes. 2. Experimental studies: • Randomized controlled trials (RCT) • Non-randomized trial Explore the association between interventions and outcomes 24-10-2023 3
  • 4. DEFINITIONS Cohort A group of individuals who have characteristics in common • Ancient Roman military unit, A band of warriors. • Persons banded together • Group of persons with a common statistical characteristic [Latin] • E.g. age, birth date 24-10-2023 4
  • 5. Examples of cohorts: • Birth cohort: All individuals in a certain geographic area born within a given period of time (usually a year) • Marriage cohort: All persons married within a given period of time • Exposure cohort: Individuals assembled as a group based on some common exposure (e.g. radiation exposure during nuclear testing, smoking exposure etc) 24-10-2023 5
  • 6. Cohort Study • A study in which two or more groups of individuals those are free of disease and those differ according to the extent of exposure to a factor of interest, are followed over a period of time to see how their exposures affect their outcomes • Presence or absence of risk factor is determined before the outcome occurs • A major limitation of cross-sectional surveys and case-control studies is difficulty in determining if exposure or risk factor preceded the disease or outcome 24-10-2023 6
  • 7. INDICATIONS OF A COHORT STUDY • When there is good evidence of exposure and disease. • When exposure is rare but incidence of disease is higher among exposed • When follow-up is easy, cohort is stable • When ample funds are available 24-10-2023 7
  • 8. FRAMEWORK OF COHORT STUDY DESIGN 24-10-2023 8
  • 10. Consideration during selection of cohort • The cohort must be free from disease under study • Insofar as the knowledge permits, both the groups should be equally susceptible to disease under study • Both the groups must be comparable in respect of all variable which influence the occurrence of disease • Diagnostic and eligibility criteria of the disease must be defined beforehand 24-10-2023 10
  • 11. TYPES OF COHORT STUDIES 1. Prospective cohort studies 2. Retrospective cohort studies 3. Ambi-directional cohort study: elements of prospective and retrospective cohort are combined. The cohort is identified from past records and assesses of date for the outcome. The same cohort is followed up prospectively into future for the further assessment of outcome. Both start by identifying subjects based upon the presence or absence of the exposure of interest, without knowing the outcome at the time their exposure status is defined 24-10-2023 11
  • 12. PROSPECTIVE COHORT STUDIES • The two groups of cohorts (exposed and un-exposed) are followed prospectively over time to track the development of new disease • Cohort characterized by determination of exposure levels (exposed vs. not exposed) at baseline (present) and followed for occurrence of disease in future • Here, groups move through time as they age 24-10-2023 12
  • 14. EXAMPLES Ramchand R, Ialongo NS, Chilcoat HD. The effect of working for pay on adolescent tobacco use. Am J Public Health. 2007 Nov;97(11):2056-62. doi: 10.2105/AJPH.2006.094045. Epub 2007 Sep 27. PMID: 17901448; PMCID: PMC2040355 • Cohort: High school students from Baltimore, Maryland • Exposure: Working for pay • Outcome: Initiation of tobacco use • Results: Adolescents who work for pay have a higher risk of initiating tobacco use 24-10-2023 14
  • 15. Seang-Mei Saw, Anoop Shankar, Say-Beng Tan, Hugh Taylor, Donald T. H. Tan, Richard A. Stone, Tien-Yin Wong; A Cohort Study of Incident Myopia in Singaporean Children. Invest. Ophthalmol. Vis. Sci. 2006;47(5):1839-1844. doi: https://doi.org/10.1167/iovs.05-1081 • Cohort: Two schools located in the northeastern and southeastern parts of Singapore, and in 2001 in one school located in the west. • Exposure: Cycloplegic refraction • Outcome: Incident myopia • Results: Parental myopia and children’s IQ are important risk factors for incident myopia in young Singaporean children 24-10-2023 15
  • 16. RETROSPECTIVE COHORT STUDIES • Makes use of historical data to determine exposure level at some baseline in the past and then determine subsequent disease status in the present • Both exposure and disease have occurred at the start of study. • Data already collected for other purposes. • The cohort is followed up retrospectively. • The study period may be many years but the time to complete the study is only as long as it takes to collate and analyze the data 24-10-2023 16
  • 18. EXAMPLE Klungel OH, Heckbert SR, de Boer A, Leufkens HG, Sullivan SD, Fishman PA, Veenstra DL, Psaty BM. Lipid-lowering drug use and cardiovascular events after myocardial infarction. Ann Pharmacother. 2002 May;36(5):751-7. doi: 10.1345/aph.1A308. PMID: 11978147 • Begin study in 2000 using data already collected via health plan. • Cohort: Surviving myocardial infarction (MI) 1986-1996 • Exposed: Lipid lowering therapy use • Outcome: Cardiovascular events during 6 months following MI 24-10-2023 18
  • 19. BASIC MEASURES Measures of disease occurrence: • Cumulative Incidence • Incidence Rate (IR) Measures of association between a factor (exposure) and a disease: • Relative Risk (RR) • Attributable Risk (AR) 24-10-2023 19
  • 20. 1. Cumulative Incidence: • Risk of developing disease • New cases of disease/persons at risk (during the same time period) – Risk of disease in exposed: a/a+b – Risk of disease in non-exposed: c/c+d Disease Non-disease Exposed a b a+b Non-exposed c d c+d a+c b+d 24-10-2023 20
  • 21. 2. Incidence Rate (IR) • Risk per unit of time • new cases of disease/Persons at risk*Duration • Example: IR, Person-time calculation, a 9-year follow-up study 24-10-2023 21
  • 22. 3. Relative Risk (RR): • Incidence of disease in exposed compared to the incidence of disease in unexposed • RR= (a/a+b)/(c/c+d) 24-10-2023 22 Disease Non-disease Exposed a b a+b Non-exposed c d c+d a+c b+d
  • 23. • Determine the strength of the association between exposure and disease • RR=1 (no association) • RR>1 (exposure increases risk for disease, e.g. RR=2.0 can be interpreted as two fold increase in risk) • RR<1 (exposure decreases risk for disease, e.g. RR=0.7 can be interpreted as 30% decrease in risk) 24-10-2023 23
  • 24. Example: (Tuberculosis treatment and breast cancer study) • Exposed=41/28011=1.5/1000 woman-years • Non-exposed=15/19025=0.8/1000 woman-years • RR=Exposed/Non-exposed=1.9 • Results: Women exposed to fluoroscopies had 1.9 times the risk of breast cancer compared to unexposed women 24-10-2023 24
  • 25. Attributable Risk (AR): • The excess risk of disease observed among exposed subjects. • AR=Exposed - Non-exposed Example: (Tuberculosis treatment and breast cancer study) • Exposed=1.5/1000 woman-years • Non-exposed=0.8/1000 woman-years • AR=Exposed - Non-exposed=1.5-0.8=0.7/1000w/y • Excess IR of breast cancer among women exposed to fluoroscopies was 0.7/1000 woman-year 24-10-2023 25
  • 26. Which clinical questions does this study design best answer? 24-10-2023 26
  • 27. ADVANTAGES • Gold standard for studying the association between risk factor and outcome • Useful for looking at multiple exposures and their interactions • Can evaluate multiple outcomes/diseases • Clear time sequence (temporal relationship between exposure and outcome) strengthens the inference about cause • Less bias due to prospective evaluation of exposures • Efficient for rare exposures • The best or only ethical way, sometimes, to do the study (situations where randomization is not possible) 24-10-2023 27
  • 28. DISADVANTAGES • Time consuming • The problem of attrition: loss of subjects (e.g. migration or death from other causes) • Unexpected changes over time: –Changes to the environment can influence the association of disease and possible cause –Changes in diagnostic criteria and methods –Changes of staff • Financial problems: lack of funding and the high costs of record keeping 24-10-2023 28
  • 30. • 9600 subjects (rural-to-urban ratio, 4800:4800) • 40 years of age or older and was carried out from 2001 through 2004. • From the cohort, 7774 subjects (rural-to-urban ratio, 3924:3850) participated in the study. • Main outcome measures were Six-year incidence of POAG and its associated risk factors. • In 6 years, incident POAG developed in 129 subjects (2.9%; 95% confidence interval [CI], 2.4e3.4; male-to-female ratio, 65:64). Baseline age was a risk factor. 24-10-2023 30

Editor's Notes

  1. Cohort population – exposed and unexposed – exposed: diseased and non diseased, unexposed: diseased and non diseased
  2. Here, the outcome is low birth weight
  3. A variable in research simply refers to a person, place thing, or a phenomenon that you are trying to measure in some way. There are independent variables and dependent variables. Independent variable: Type of treatment: low dose of the new medication, high dose of the new medication and placebo Dependent variable: Blood pressure: No change in BP, BP is lowered, no change in BP with respect to the type of treatment.
  4. Duration (Person-time): sum of time at risk for all individuals (time until the date of the event of interest or date of censoring, i.e. death, end of FU, drop out). e.g.1 person FU for 2 years=2 person-year. Persons “at risk” who do not have the disease of interest and are capable of developing the disease. Person time: 2.1+4.8+3.2+9.0+7.2=26.3 years Incidence rate: 2 events/26.3 person-years=0.076/year (or 76/1000/year)