Cohort Studies - Week 7 IPHY

Different types of Cohort Studies

Population-based, exposure-based, prospective, retrospective

Population-Based Cohort Description

cohort includes either entire population or representative sample of the population; exposures are unknown until first period of observation when exposure information is collected

Exposure-Based Cohort Description

made up of subjects with common exposure; non-exposed group or comparison group where group is similar in demographics and geography to exposed group but lacks exposure

Prospective-Cohort Description

characterized by determination of exposure levels at baseline (present); follow up for occurrence of disease at some point in future

Retrospective-Cohort Description

investigators have to wait for cases to accrue; make use of historical data to determine exposure level at some baseline in past

Absolute Measures of Effect

risk difference

Relative Measures of Effect

relative risk/risk ratio; incidence rate ratio; odds ratio

Measures of Impact

etiologic fraction; population risk difference

What are absolute measures used for?

subtract disease frequencies from one another; gives information about the public health impact of an exposure

What are relative measures used for?

divide frequencies from one another; give information about the strength of the relationship between exposure and disease

Definition of Risk Difference

identifies number of cases of disease that would be eliminated in exposed group if exposure was eliminated

Definition of Relative Risk (risk ratio)

tells if risk of disease is different among exposed as compared to non exposed

Definition of Incidence Rate Ratio

tells if rate of disease is different among exposed compared to non exposed

Definition of Odds Ratio

tells if odds of disease are different among exposed compared to non exposed

Definition of Etiologic Fraction

proportion of rate of disease in exposed group due to exposure; tells how much particular exposure accounts for disease etiology in exposed group

Definition of Population Risk Difference

tells number of cases of disease that would be eliminated in total population if exposure was eliminated; helps public officials determine which exposures are most important to given population and helps prioritize prevention activities

Interpretation of Relative Risk (risk ratio)

exposed have # times the risk of disease compared to non exposed
the risk of disease is _% higher/lower among exposed compared to non exposed

Interpretation of Incidence Rate Ratio

exposed have # times the rate of disease compared to non exposed
the rate of disease is _% higher/lower among exposed compared to non exposed

Interpretation of Risk Difference

exposure results in an excess of # cases of disease in exposed group
if exposure were eliminated, there would be # fewer cases of disease in the exposed group

Interpretation of Etiologic Fraction

exposure accounts for _% of disease in the exposed

Interpretation of Population Risk Difference

exposure results in an excess of # cases of disease in the total population
if exposure were eliminated, there would be # fewer cases of disease in the total population

Strengths of a Cohort Study

permit direct determination or risk; time sequencing of exposure and outcome; can study multiple outcomes; can study rare exposures

Limitations of a Cohort Study

takes a long time; subjects lost to follow up; costly; selection bias; difficult to use with rare diseases

How to turn a number into a percentage

Y>1: (Y-1) x 100 = % increased
Y<1: (1-Y) x 100 = % decreased