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Prospective StudyCohort Study

Diaa Marzouk

of

Community Medicine

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Case Report

Case Control

Cohort Study

Randomized Controlled Trial

Practice Guidelines

Systematic

Review

Meta

Analysis

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Case Reports

An article that describes and interprets anindividual case, often written in the form of adetailed story. Case reports often describe:

Unique cases that cannot be explained by knowndiseases or syndromes

Cases that show an important variation of adisease or condition

Cases that show unexpected events that mayyield new or useful information

Cases in which one patient has two or moreunexpected diseases or disorders

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Case Reports

Case reports are considered the lowest level ofevidence, but they are also the first line ofevidence, because they are where new issues andideas emerge.

This is why they form the base of our pyramid. Agood case report will be clear about theimportance of the observation being reported.

If multiple case reports show something similar,

the next step might be a case-control study todetermine if there is a relationship between therelevant variables.

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Case Reports (example)

Wu, E. B., & Sung, J. J. Y. (2003). Haemorrhagic-fever-like changes and normal chest radiograph in a doctorwith SARS. Lancet, 361(9368), 1520-1521.

This case report is written by the patient, a physicianwho contracted SARS, and his colleague who treatedhim, during the 2003 outbreak of SARS in Hong Kong.They describe how the disease progressed in Dr. Wuand based on Dr. Wus case, advised that a chest CT

showed hidden pneumonic changes and facilitate arapid diagnosis.

http://www.ncbi.nlm.nih.gov/sites/entrez?orig_db=PubMed&db=pubmed&cmd=Search&TransSchema=title&term=Haemorrhagic-fever-like%20changes%20and%20normal%20chest%20radiograph%20in%20a%20doctor%20with%20SARShttp://www.ncbi.nlm.nih.gov/sites/entrez?orig_db=PubMed&db=pubmed&cmd=Search&TransSchema=title&term=Haemorrhagic-fever-like%20changes%20and%20normal%20chest%20radiograph%20in%20a%20doctor%20with%20SARShttp://www.ncbi.nlm.nih.gov/sites/entrez?orig_db=PubMed&db=pubmed&cmd=Search&TransSchema=title&term=Haemorrhagic-fever-like%20changes%20and%20normal%20chest%20radiograph%20in%20a%20doctor%20with%20SARShttp://www.ncbi.nlm.nih.gov/sites/entrez?orig_db=PubMed&db=pubmed&cmd=Search&TransSchema=title&term=Haemorrhagic-fever-like%20changes%20and%20normal%20chest%20radiograph%20in%20a%20doctor%20with%20SARShttp://www.ncbi.nlm.nih.gov/sites/entrez?orig_db=PubMed&db=pubmed&cmd=Search&TransSchema=title&term=Haemorrhagic-fever-like%20changes%20and%20normal%20chest%20radiograph%20in%20a%20doctor%20with%20SARShttp://www.ncbi.nlm.nih.gov/sites/entrez?orig_db=PubMed&db=pubmed&cmd=Search&TransSchema=title&term=Haemorrhagic-fever-like%20changes%20and%20normal%20chest%20radiograph%20in%20a%20doctor%20with%20SARShttp://www.ncbi.nlm.nih.gov/sites/entrez?orig_db=PubMed&db=pubmed&cmd=Search&TransSchema=title&term=Haemorrhagic-fever-like%20changes%20and%20normal%20chest%20radiograph%20in%20a%20doctor%20with%20SARShttp://www.ncbi.nlm.nih.gov/sites/entrez?orig_db=PubMed&db=pubmed&cmd=Search&TransSchema=title&term=Haemorrhagic-fever-like%20changes%20and%20normal%20chest%20radiograph%20in%20a%20doctor%20with%20SARShttp://www.ncbi.nlm.nih.gov/sites/entrez?orig_db=PubMed&db=pubmed&cmd=Search&TransSchema=title&term=Haemorrhagic-fever-like%20changes%20and%20normal%20chest%20radiograph%20in%20a%20doctor%20with%20SARShttp://www.ncbi.nlm.nih.gov/sites/entrez?orig_db=PubMed&db=pubmed&cmd=Search&TransSchema=title&term=Haemorrhagic-fever-like%20changes%20and%20normal%20chest%20radiograph%20in%20a%20doctor%20with%20SARShttp://www.ncbi.nlm.nih.gov/sites/entrez?orig_db=PubMed&db=pubmed&cmd=Search&TransSchema=title&term=Haemorrhagic-fever-like%20changes%20and%20normal%20chest%20radiograph%20in%20a%20doctor%20with%20SARShttp://www.ncbi.nlm.nih.gov/sites/entrez?orig_db=PubMed&db=pubmed&cmd=Search&TransSchema=title&term=Haemorrhagic-fever-like%20changes%20and%20normal%20chest%20radiograph%20in%20a%20doctor%20with%20SARShttp://www.ncbi.nlm.nih.gov/sites/entrez?orig_db=PubMed&db=pubmed&cmd=Search&TransSchema=title&term=Haemorrhagic-fever-like%20changes%20and%20normal%20chest%20radiograph%20in%20a%20doctor%20with%20SARShttp://www.ncbi.nlm.nih.gov/sites/entrez?orig_db=PubMed&db=pubmed&cmd=Search&TransSchema=title&term=Haemorrhagic-fever-like%20changes%20and%20normal%20chest%20radiograph%20in%20a%20doctor%20with%20SARShttp://www.ncbi.nlm.nih.gov/sites/entrez?orig_db=PubMed&db=pubmed&cmd=Search&TransSchema=title&term=Haemorrhagic-fever-like%20changes%20and%20normal%20chest%20radiograph%20in%20a%20doctor%20with%20SARShttp://www.ncbi.nlm.nih.gov/sites/entrez?orig_db=PubMed&db=pubmed&cmd=Search&TransSchema=title&term=Haemorrhagic-fever-like%20changes%20and%20normal%20chest%20radiograph%20in%20a%20doctor%20with%20SARShttp://www.ncbi.nlm.nih.gov/sites/entrez?orig_db=PubMed&db=pubmed&cmd=Search&TransSchema=title&term=Haemorrhagic-fever-like%20changes%20and%20normal%20chest%20radiograph%20in%20a%20doctor%20with%20SARS

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Case Control Study

A study that compares patients who have adisease or outcome of interest (cases) withpatients who do not have the disease or

outcome (controls), and looks back retrospectively to compare how

frequently the exposure to a risk factor ispresent in each group to determine therelationship between the risk factor and thedisease.

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Case Control Study (example)

Teo, K. K., Ounpuu, S., Hawken, S., Pandey, M.,Valentin, V., & Hunt, D. et al. (2006). Tobacco useand risk of myocardial infarction in 52 countries

in the INTERHEART study: A case-control study.Lancet, 368(9536), 647-658.

This study looked at the relation between risk ofacute myocardial infarction and current or former

smoking, type of tobacco, amount smoked, effectof smokeless tobacco, and exposure tosecondhand smoke.

http://www.ncbi.nlm.nih.gov/pubmed/16920470?ordinalpos=2&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSumhttp://www.ncbi.nlm.nih.gov/pubmed/16920470?ordinalpos=2&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSumhttp://www.ncbi.nlm.nih.gov/pubmed/16920470?ordinalpos=2&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSumhttp://www.ncbi.nlm.nih.gov/pubmed/16920470?ordinalpos=2&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSumhttp://www.ncbi.nlm.nih.gov/pubmed/16920470?ordinalpos=2&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSumhttp://www.ncbi.nlm.nih.gov/pubmed/16920470?ordinalpos=2&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSumhttp://www.ncbi.nlm.nih.gov/pubmed/16920470?ordinalpos=2&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSumhttp://www.ncbi.nlm.nih.gov/pubmed/16920470?ordinalpos=2&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSumhttp://www.ncbi.nlm.nih.gov/pubmed/16920470?ordinalpos=2&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSumhttp://www.ncbi.nlm.nih.gov/pubmed/16920470?ordinalpos=2&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSumhttp://www.ncbi.nlm.nih.gov/pubmed/16920470?ordinalpos=2&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSumhttp://www.ncbi.nlm.nih.gov/pubmed/16920470?ordinalpos=2&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSumhttp://www.ncbi.nlm.nih.gov/pubmed/16920470?ordinalpos=2&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSumhttp://www.ncbi.nlm.nih.gov/pubmed/16920470?ordinalpos=2&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSumhttp://www.ncbi.nlm.nih.gov/pubmed/16920470?ordinalpos=2&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSumhttp://www.ncbi.nlm.nih.gov/pubmed/16920470?ordinalpos=2&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSumhttp://www.ncbi.nlm.nih.gov/pubmed/16920470?ordinalpos=2&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSumhttp://www.ncbi.nlm.nih.gov/pubmed/16920470?ordinalpos=2&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSumhttp://www.ncbi.nlm.nih.gov/pubmed/16920470?ordinalpos=2&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSumhttp://www.ncbi.nlm.nih.gov/pubmed/16920470?ordinalpos=2&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSumhttp://www.ncbi.nlm.nih.gov/pubmed/16920470?ordinalpos=2&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSumhttp://www.ncbi.nlm.nih.gov/pubmed/16920470?ordinalpos=2&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSumhttp://www.ncbi.nlm.nih.gov/pubmed/16920470?ordinalpos=2&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSumhttp://www.ncbi.nlm.nih.gov/pubmed/16920470?ordinalpos=2&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSumhttp://www.ncbi.nlm.nih.gov/pubmed/16920470?ordinalpos=2&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSum

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Prospective Study /

Cohort Study

A study design that follows prospectively overtime one or more populations (called cohorts)to determine which patient characteristics

(risk factors) are associated with thedevelopment of a disease or outcome.

As the study is conducted, the outcome fromparticipants in each cohort is measured andrelationships with specific characteristicsdetermined.

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Randomized Clinical Trials

A study design that randomly assigns

participants into an experimental group or a

control group.

As the study is conducted, the only expected

difference between the control and

experimental groups in a randomized

controlled trial (RCT) is the outcome variablebeing studied.

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Randomized Clinical Trials (example)

Krishna, S., Balas, E. A., Francisco, B. D., & Knig, P. (2006). Effectiveand sustainable multimedia education for children with asthma: Arandomized controlled trial. Children's Health Care, 35(1), 75-90.

This article reports on the effect of multimedia education forchildren with asthma.

A control group of pediatric patients with asthma was givenstandard asthma educational resources,

while the experimental group of pediatric patients withasthma was given standard resources plus multimediaresources.

The study found a reduction in daily symptoms, in emergencyroom visits, in school days missed, and in days of limitedactivity in the group given multimedia education resources.

http://www.informaworld.com/smpp/content~content=a783764394http://www.informaworld.com/smpp/content~content=a783764394http://www.informaworld.com/smpp/content~content=a783764394http://www.informaworld.com/smpp/content~content=a783764394http://www.informaworld.com/smpp/content~content=a783764394http://www.informaworld.com/smpp/content~content=a783764394http://www.informaworld.com/smpp/content~content=a783764394http://www.informaworld.com/smpp/content~content=a783764394http://www.informaworld.com/smpp/content~content=a783764394http://www.informaworld.com/smpp/content~content=a783764394http://www.informaworld.com/smpp/content~content=a783764394http://www.informaworld.com/smpp/content~content=a783764394http://www.informaworld.com/smpp/content~content=a783764394http://www.informaworld.com/smpp/content~content=a783764394http://www.informaworld.com/smpp/content~content=a783764394http://www.informaworld.com/smpp/content~content=a783764394http://www.informaworld.com/smpp/content~content=a783764394

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Practice Guidelines

A statement produced by a panel of expertsthat outlines current best practice to informhealth care professionals and patients in

making clinical decisions. It is produced after an extensive review of the

literature and

is typically created by professionalassociations, government agencies, and/orpublic or private organizations.

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Systematic Review

A document often written by a panel thatprovides a comprehensive review of all relevantstudies on a particular clinical or health-relatedtopic/question.

It is created after reviewing and combining all theinformation from both published andunpublished studies

(focusing on clinical trials of similar treatments)and

then summarizing the findings.

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Systematic Review (example)

Pittler MH, Guo R, Ernst E. Hawthorn extract fortreating chronic heart failure. Cochrane Database ofSystematic Reviews 2008, Issue 1. Art. No.: CD005312.

This systematic review analyzed fourteen studies

(randomized, double-blinded, and placebo controlled)that used hawthorn leaf and flower extractmonopreparations to determine whether there is anybenefit or harm in using hawthorn extract to treatchronic heart failure when compared to placebo. The

authors determined that hawthorn extract providessignificant benefit in symptom control and physiologicoutcomes when used as an adjuvant treatment forchronic heart failure.

http://www.ncbi.nlm.nih.gov/pubmed/18254076?ordinalpos=1&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSumhttp://www.ncbi.nlm.nih.gov/pubmed/18254076?ordinalpos=1&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSumhttp://www.ncbi.nlm.nih.gov/pubmed/18254076?ordinalpos=1&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSumhttp://www.ncbi.nlm.nih.gov/pubmed/18254076?ordinalpos=1&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSumhttp://www.ncbi.nlm.nih.gov/pubmed/18254076?ordinalpos=1&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSumhttp://www.ncbi.nlm.nih.gov/pubmed/18254076?ordinalpos=1&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSumhttp://www.ncbi.nlm.nih.gov/pubmed/18254076?ordinalpos=1&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSumhttp://www.ncbi.nlm.nih.gov/pubmed/18254076?ordinalpos=1&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSumhttp://www.ncbi.nlm.nih.gov/pubmed/18254076?ordinalpos=1&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSumhttp://www.ncbi.nlm.nih.gov/pubmed/18254076?ordinalpos=1&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSumhttp://www.ncbi.nlm.nih.gov/pubmed/18254076?ordinalpos=1&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSum

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Meta-Analysis

A method for systematically combining tqualitative and quantitative study data fromseveral selected studies to develop a single

conclusion that has greater statistical power. This conclusion is statistically stronger than

the analysis of any single study, due toincreased numbers of subjects, greaterdiversity among subjects, or accumulatedeffects and results.

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Meta-Analysis

If the individual studies utilized randomized

controlled trials (RCT), combining several

selected RCT results would be the highest-

level of evidence on the evidence hierarchy,followed by systematic reviews, which analyze

all available studies on a topic.

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Meta-Analysis (example)

Ageno, W., Becattini, C., Brighton, T., Selby, R., Kamphuisen, P.W. (2008). Cardiovascular risk factors and venousthromboembolism: A meta-analysis. Circulation, 117(1), 93-102.

The concept that venous thromboembolism (VTE) and atherosclerosis aretwo completely distinct entities has recently been challenged becausepatients with VTE have more asymptomatic atherosclerosis and morecardiovascular events than control subjects using meta-analysistechniques to assess the association between cardiovascular risk factorsand VTE.

Twenty-one case-control and cohort studies with a total of 63,552 patientswere included, showing the risk of VTE was 2.33 for obesity (95% CI, 1.68to 3.24), 1.51 for hypertension (95% CI, 1.23 to 1.85), 1.42 for diabetesmellitus (95% CI, 1.12 to 1.77), 1.18 for smoking (95% CI, 0.95 to 1.46), and1.16 for hypercholesterolemia (95% CI, 0.67 to 2.02).

This demonstrated that cardiovascular risk factors are associated with VTE,which is clinically relevant with respect to individual screening, risk factormodification, and primary and secondary prevention of VTE. Prospectivestudies should further investigate the underlying mechanisms of thisrelationship.

http://www.ncbi.nlm.nih.gov/pubmed/18086925?ordinalpos=4&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSumhttp://www.ncbi.nlm.nih.gov/pubmed/18086925?ordinalpos=4&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSumhttp://www.ncbi.nlm.nih.gov/pubmed/18086925?ordinalpos=4&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSumhttp://www.ncbi.nlm.nih.gov/pubmed/18086925?ordinalpos=4&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSumhttp://www.ncbi.nlm.nih.gov/pubmed/18086925?ordinalpos=4&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSumhttp://www.ncbi.nlm.nih.gov/pubmed/18086925?ordinalpos=4&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSumhttp://www.ncbi.nlm.nih.gov/pubmed/18086925?ordinalpos=4&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSumhttp://www.ncbi.nlm.nih.gov/pubmed/18086925?ordinalpos=4&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSumhttp://www.ncbi.nlm.nih.gov/pubmed/18086925?ordinalpos=4&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSumhttp://www.ncbi.nlm.nih.gov/pubmed/18086925?ordinalpos=4&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSumhttp://www.ncbi.nlm.nih.gov/pubmed/18086925?ordinalpos=4&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSumhttp://www.ncbi.nlm.nih.gov/pubmed/18086925?ordinalpos=4&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSumhttp://www.ncbi.nlm.nih.gov/pubmed/18086925?ordinalpos=4&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSumhttp://www.ncbi.nlm.nih.gov/pubmed/18086925?ordinalpos=4&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSumhttp://www.ncbi.nlm.nih.gov/pubmed/18086925?ordinalpos=4&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSumhttp://www.ncbi.nlm.nih.gov/pubmed/18086925?ordinalpos=4&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSumhttp://www.ncbi.nlm.nih.gov/pubmed/18086925?ordinalpos=4&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSumhttp://www.ncbi.nlm.nih.gov/pubmed/18086925?ordinalpos=4&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSumhttp://www.ncbi.nlm.nih.gov/pubmed/18086925?ordinalpos=4&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSum

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Cohort Study

It starts with a group of

people / cohort:

All considered to be free of a

given disease.

But vary in exposure to asupposed risk factor.

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Cohort Study (example 1)

Nichol, K. L., Nordin, J. D., Nelson, D. B., Mullooly, J. P., & Hak, E.(2007). Effectiveness of influenza vaccine in the community-dwelling elderly. New England Journal of Medicine, 357(14), 1373-1381.

To determine the long-term effectiveness ofinfluenza vaccines in elderly people, cohortsof vaccinated elderly and unvaccinatedcommunity-dwelling elderly were studied. The

results suggest that the elderly who arevaccinated have a reduced risk ofhospitalization for pneumonia or influenza.

http://www.ncbi.nlm.nih.gov/sites/entrez?orig_db=PubMed&db=pubmed&cmd=Search&TransSchema=title&term=Effectiveness%20of%20influenza%20vaccine%20in%20the%20community-dwelling%20elderlyhttp://www.ncbi.nlm.nih.gov/sites/entrez?orig_db=PubMed&db=pubmed&cmd=Search&TransSchema=title&term=Effectiveness%20of%20influenza%20vaccine%20in%20the%20community-dwelling%20elderlyhttp://www.ncbi.nlm.nih.gov/sites/entrez?orig_db=PubMed&db=pubmed&cmd=Search&TransSchema=title&term=Effectiveness%20of%20influenza%20vaccine%20in%20the%20community-dwelling%20elderlyhttp://www.ncbi.nlm.nih.gov/sites/entrez?orig_db=PubMed&db=pubmed&cmd=Search&TransSchema=title&term=Effectiveness%20of%20influenza%20vaccine%20in%20the%20community-dwelling%20elderlyhttp://www.ncbi.nlm.nih.gov/sites/entrez?orig_db=PubMed&db=pubmed&cmd=Search&TransSchema=title&term=Effectiveness%20of%20influenza%20vaccine%20in%20the%20community-dwelling%20elderlyhttp://www.ncbi.nlm.nih.gov/sites/entrez?orig_db=PubMed&db=pubmed&cmd=Search&TransSchema=title&term=Effectiveness%20of%20influenza%20vaccine%20in%20the%20community-dwelling%20elderlyhttp://www.ncbi.nlm.nih.gov/sites/entrez?orig_db=PubMed&db=pubmed&cmd=Search&TransSchema=title&term=Effectiveness%20of%20influenza%20vaccine%20in%20the%20community-dwelling%20elderlyhttp://www.ncbi.nlm.nih.gov/sites/entrez?orig_db=PubMed&db=pubmed&cmd=Search&TransSchema=title&term=Effectiveness%20of%20influenza%20vaccine%20in%20the%20community-dwelling%20elderlyhttp://www.ncbi.nlm.nih.gov/sites/entrez?orig_db=PubMed&db=pubmed&cmd=Search&TransSchema=title&term=Effectiveness%20of%20influenza%20vaccine%20in%20the%20community-dwelling%20elderlyhttp://www.ncbi.nlm.nih.gov/sites/entrez?orig_db=PubMed&db=pubmed&cmd=Search&TransSchema=title&term=Effectiveness%20of%20influenza%20vaccine%20in%20the%20community-dwelling%20elderlyhttp://www.ncbi.nlm.nih.gov/sites/entrez?orig_db=PubMed&db=pubmed&cmd=Search&TransSchema=title&term=Effectiveness%20of%20influenza%20vaccine%20in%20the%20community-dwelling%20elderlyhttp://www.ncbi.nlm.nih.gov/sites/entrez?orig_db=PubMed&db=pubmed&cmd=Search&TransSchema=title&term=Effectiveness%20of%20influenza%20vaccine%20in%20the%20community-dwelling%20elderlyhttp://www.ncbi.nlm.nih.gov/sites/entrez?orig_db=PubMed&db=pubmed&cmd=Search&TransSchema=title&term=Effectiveness%20of%20influenza%20vaccine%20in%20the%20community-dwelling%20elderlyhttp://www.ncbi.nlm.nih.gov/sites/entrez?orig_db=PubMed&db=pubmed&cmd=Search&TransSchema=title&term=Effectiveness%20of%20influenza%20vaccine%20in%20the%20community-dwelling%20elderlyhttp://www.ncbi.nlm.nih.gov/sites/entrez?orig_db=PubMed&db=pubmed&cmd=Search&TransSchema=title&term=Effectiveness%20of%20influenza%20vaccine%20in%20the%20community-dwelling%20elderlyhttp://www.ncbi.nlm.nih.gov/sites/entrez?orig_db=PubMed&db=pubmed&cmd=Search&TransSchema=title&term=Effectiveness%20of%20influenza%20vaccine%20in%20the%20community-dwelling%20elderly

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Cohort Study (example 2)

Ramchand, R., Ialongo, N. S., & Chilcoat, H. D. (2007).The effect of working for pay on adolescent tobaccouse.American Journal of Public Health, 97(11), 2056-2062.

This study uses data collected from high schoolstudents from Baltimore, Maryland, and studies thedifferences in initiation of tobacco use between acohort of adolescents that started working for pay and

a cohort of adolescents that did not work. The resultssuggest that adolescents who work for pay have ahigher risk of initiating tobacco use.

http://www.ncbi.nlm.nih.gov/sites/entrez?orig_db=PubMed&db=pubmed&cmd=Search&TransSchema=title&term=The%20effect%20of%20working%20for%20pay%20on%20adolescent%20tobacco%20usehttp://www.ncbi.nlm.nih.gov/sites/entrez?orig_db=PubMed&db=pubmed&cmd=Search&TransSchema=title&term=The%20effect%20of%20working%20for%20pay%20on%20adolescent%20tobacco%20usehttp://www.ncbi.nlm.nih.gov/sites/entrez?orig_db=PubMed&db=pubmed&cmd=Search&TransSchema=title&term=The%20effect%20of%20working%20for%20pay%20on%20adolescent%20tobacco%20usehttp://www.ncbi.nlm.nih.gov/sites/entrez?orig_db=PubMed&db=pubmed&cmd=Search&TransSchema=title&term=The%20effect%20of%20working%20for%20pay%20on%20adolescent%20tobacco%20usehttp://www.ncbi.nlm.nih.gov/sites/entrez?orig_db=PubMed&db=pubmed&cmd=Search&TransSchema=title&term=The%20effect%20of%20working%20for%20pay%20on%20adolescent%20tobacco%20usehttp://www.ncbi.nlm.nih.gov/sites/entrez?orig_db=PubMed&db=pubmed&cmd=Search&TransSchema=title&term=The%20effect%20of%20working%20for%20pay%20on%20adolescent%20tobacco%20usehttp://www.ncbi.nlm.nih.gov/sites/entrez?orig_db=PubMed&db=pubmed&cmd=Search&TransSchema=title&term=The%20effect%20of%20working%20for%20pay%20on%20adolescent%20tobacco%20usehttp://www.ncbi.nlm.nih.gov/sites/entrez?orig_db=PubMed&db=pubmed&cmd=Search&TransSchema=title&term=The%20effect%20of%20working%20for%20pay%20on%20adolescent%20tobacco%20usehttp://www.ncbi.nlm.nih.gov/sites/entrez?orig_db=PubMed&db=pubmed&cmd=Search&TransSchema=title&term=The%20effect%20of%20working%20for%20pay%20on%20adolescent%20tobacco%20usehttp://www.ncbi.nlm.nih.gov/sites/entrez?orig_db=PubMed&db=pubmed&cmd=Search&TransSchema=title&term=The%20effect%20of%20working%20for%20pay%20on%20adolescent%20tobacco%20usehttp://www.ncbi.nlm.nih.gov/sites/entrez?orig_db=PubMed&db=pubmed&cmd=Search&TransSchema=title&term=The%20effect%20of%20working%20for%20pay%20on%20adolescent%20tobacco%20usehttp://www.ncbi.nlm.nih.gov/sites/entrez?orig_db=PubMed&db=pubmed&cmd=Search&TransSchema=title&term=The%20effect%20of%20working%20for%20pay%20on%20adolescent%20tobacco%20usehttp://www.ncbi.nlm.nih.gov/sites/entrez?orig_db=PubMed&db=pubmed&cmd=Search&TransSchema=title&term=The%20effect%20of%20working%20for%20pay%20on%20adolescent%20tobacco%20usehttp://www.ncbi.nlm.nih.gov/sites/entrez?orig_db=PubMed&db=pubmed&cmd=Search&TransSchema=title&term=The%20effect%20of%20working%20for%20pay%20on%20adolescent%20tobacco%20usehttp://www.ncbi.nlm.nih.gov/sites/entrez?orig_db=PubMed&db=pubmed&cmd=Search&TransSchema=title&term=The%20effect%20of%20working%20for%20pay%20on%20adolescent%20tobacco%20usehttp://www.ncbi.nlm.nih.gov/sites/entrez?orig_db=PubMed&db=pubmed&cmd=Search&TransSchema=title&term=The%20effect%20of%20working%20for%20pay%20on%20adolescent%20tobacco%20usehttp://www.ncbi.nlm.nih.gov/sites/entrez?orig_db=PubMed&db=pubmed&cmd=Search&TransSchema=title&term=The%20effect%20of%20working%20for%20pay%20on%20adolescent%20tobacco%20usehttp://www.ncbi.nlm.nih.gov/sites/entrez?orig_db=PubMed&db=pubmed&cmd=Search&TransSchema=title&term=The%20effect%20of%20working%20for%20pay%20on%20adolescent%20tobacco%20usehttp://www.ncbi.nlm.nih.gov/sites/entrez?orig_db=PubMed&db=pubmed&cmd=Search&TransSchema=title&term=The%20effect%20of%20working%20for%20pay%20on%20adolescent%20tobacco%20use

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What is a cohort?

A cohort is a group of

persons who share a common

experience within a definedtime period.

Healthy cohorts are followedup forward in time for the

development of a disease

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Examples of cohorts:

Birth cohort:

All persons born within a

given period of time.Marriage cohort:

All persons married within agiven period of time

Occupational cohort:

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Selection of cohorts

Volunteers:

Because it is acssesable

From armed forces:

Because its medical recordsare available

Have particular exposure:

At work

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Cohort Study

Information is gathered

about their exposure to the

suspected risk factor

The individuals are divided

into exposed and not exposedTo the factor of interest

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Cohort Study

The cohort is followed over

time in order to determine

differences in the rate atwhich disease develops in

relation to exposure to the

factor

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Advantages of Prospective Study

1. The cohort is classified inrelation to exposure to the

factor before the diseasedevelops

This classification cannot

be influenced by theknowledge that diseaseexists.

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Advantages

2. It permits calculation of

Incidence rates among

exposed and not exposed.

3. The absolute difference

between both is the

attributable risk.

4. The relative risk.

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Advantages

5. Permits observation of manyoutcomes,

e.g smoking and lung cancer,showed that smoking isassociated with other otcomesas emphysema, CHD, peptic

ulcer, cancer larynx, oral cavity,oesophagus and urinary bladder.

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Disadvantages of Prospective

studies

1. Long, expensive and large

scale undertaking.

2. The problem of attrition:

loss of patients due to lack

of interest, migration or

death from other causes.

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Disadvantages

3. Changes in diagnostic

criteria and methods over

time

4. Administrative problems as

loss of staff, loss of funding,

and the high costs of record

keeping

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Analysis of Results

Anlaytic studies are designed

to determine whether an

association exists between afactor or exposure and a

disease and to determine the

strength of the association.

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Relative Risk

It is an important measure of

association that relates the

incidence rates of thedisease under study among

those with and without the

factor or exposure.

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Relative Risk

It is defined as the ratio of

the incidence rate for

persons exposed to theincidence rate for those not

exposed.

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Relative Risk (RR)=

Incidence among exposed

Incidence among unexposed

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Diseased Not Diseased

Exposed a b

Not Exposed c d

RR= a c = ada+b c+d bc

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NoPneumonia /

InfluenzaPneumonia /

Influenza

Vaccinated elderly 80 (a) 40 (b)

Non Vaccinated

elderly

20 (c) 60 (d)

Total 100 100

RR= a c = ad

a+b c+d bc80/100 20/80 = 80 60 / 40 20 = 6

RR = 6

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Historical Prospective Studies

It combines the advantages

of both retrospective and

prospective study desgins.

It involves following healthy

exposed and unexposed

cohorts for the development

of the disease.

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These cohorts are

constructed retrospectively

through existing records thatpermit correct classification

of the exposure status of

individuals

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Study subjectd are traced to

the present time or

sometimes to the future aswell.

The analysis of this study is

as the prospective study.

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