Cohort studies
Measures of Outcome
Incidence rate
Cumulative incidence/Incidence proportion [risk]
Per 1000 people
No. of people at risk of the disease at the start of the period
Time period refers to the time an individual remains free of the disease.
Expressed>no. of new cases/total person-years of observation.
No. of people who get the disease (new cases) in a specified period.
Measures of association btwn exposure & outcome
Absolute Comparison
Relative Comparison
Measures ratios=e.g Risk ratios,relative risk
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Relative risk
Clinic relevance=indicates individual risk
compares the risk among the exposed to the risk among the unexposed
Measures difference=Risk differences, attributable risk
Attributable risk
Measure the difference in frequency of disease (risk) btwn the exposed and unexposed groups
indicates the risk of disease in an exposed group caused(attributed) by the exposure
Clinic relevance=measure extent of a disease caused by an exposure
Interpretation of Relative Risk
Exposure=Risk factor?
Risk Comparison
RR=1:No Difference btwn exposed & unexposed
RR>1Risk of disease is higher in exposed than in unexposed group
RR<1:risk of disease in exposed group is lower than in unexposed
RR<1:Reduces risk factor>Protective=
RR=1:No
RR>1:Increases risk factor
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Attributable Risk interpretation
Relative Risk interpretation
Implications: Significant=no overlapping 1 , non-significant=overlaps 1
Confidence intervals=check precision of Risk estimate
AR of 0 means no difference so no additional risk caused by that exposure
Predict burden on health services