COHORT STUDIES.pptx

This ppt provides in detail about the cohort study, its types and with examples

1. COHORT STUDIES Submitted by, Reshma S Suresh

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

9. Population: Pregnant women Exposure:Smokers Exposure:Non-Smokers <2.5Kg (LBW) ≥ 2.5Kg <2.5Kg (LBW) ≥2.5Kg 24-10-2023 9

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 Prospective cohort studies Retrospective cohort studies 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

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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 : C ycloplegic 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

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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 N ew 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) E xposed=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= E xposed - Non-exposed Example: (Tuberculosis treatment and breast cancer study) E xposed=1.5/1000 woman-years N on-exposed=0.8/1000 woman-years AR= E xposed - N on-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

29. OTHER EXAMPLES 24-10-2023 29

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

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