Measures of Effect Flashcards
What is the measure of effect?
Measure of Effect:
A quantity that expresses the strength of association between two variables, often referred to as the exposure variable and disease variable in epidemiologic research; a comparison of two or more measures of disease frequency.
Synonyms: Effect Measure, Measure of Association.
What are two measures of effect?
The odds ratio (case-control studies) and the risk ratio (cohort studies).
What is the formula for risk ratio (RR)
^RR = Est. Risk for group 1/Est. risk for group 2
How would you interpret a risk ratio of 1.1?
The difference is not meaningful because it is very close to 1.0 (null risk) - which means there is no meaningful difference between the two risk groups.
Why can we not use risk ratios for case control studies?
We cannot estimate risk because we are working with and comparing existing cases. The proportions of existing cases to exposures do not tell us risk; therefore you cannot identify the risk ratio.
What measure of effect is effective for case-control studies and why?
The Odds Ratio is most effective because it gives a ratio of the odds of cases having a certain exposure in contrast with the odds of the control also having the same exposure.
What is the formula for odds?
O = P/1-P
where P is the probability of the event.
How do we calculate odds ratio (OR)
OR = Odds of Cases/Odds of Controls
It can also save time to use the cross product ratio formula of OR = ad/bc
What are the different names for the odds ratio by study?
Case Control: Exposure odds ratio (EOR)
(Because it measures past exposure probability)
Follow-up (Cohort): Risk odds ratio (ROR)
(Because it measures the risk)
Cross Sectional: Prevalence odds ratio (POR)
(because this study measures the prevalence of a health outcome)
In which study can you use both risk ratio and odds ratio? What relationship do they share in this study?
Cohort/Follow-up Study
If the RR is less than 1, so is the ROR
If the RR is greater than 1, so is the ROR
When is a disease considered to be rare?
When the true risk is equal to 0. This can be in the case of 0.001 or 0.0001. Sometimes .01 can be an instant that is not rare, but is a low number due to the short period of follow-up for a chronic disease.
What is the rare disease assumption?
Rare Disease Assumption:
Assumes that the risk that any individual in the population being studied is very small (i.e., approximately zero); in follow-up studies, the ROR approximates the RR if the rare disease assumption holds; in case-control studies, the EOR estimates an RR from a comparable follow-up study if the rare disease assumption holds, the cases are incident cases, and the controls are representative of the source population.
What is EOR? When do you use it?
EOR:
Exposure Odds Ratio; the odds ratio computed in case-control studies.
What is the rate ratio? When do you use it?
Rate ratio:
A measure of effect defined as the ratio of two rates; synonyms: incidence density ratio (IDR); hazard ratio.
IDR is used in cohort studies in which person-time information is obtained
What is the IDR Formula?
The rate ratio (IDR) formula:
IDR = (Incidence1/Person Time1)/(Incidence0/Persontime0). When assessing two groups.
