Biostats_3_Relative Risk vs Odds Ratio Flashcards
What makes the relative risk and odds ratio similar?
Relative risk and odds ratio are measures of association which provide point estimates of effect. They are useful in describing the magnitude of an effect.
What is typically used in studies where participants are followed prospectively to observe outcomes?
What type of study is this?
Relative Risk is used in a cohort study where people are followed and their “risk” of developing a disease later in the future refers to the probability of disease (or an event) occurring over a certain period of time. These are prospective study designs.
When two groups of interest are compared in a study over time, how (generally) is the ratio set up that functions to compare the group at risk vs the group not at risk ?
A ratio, called “relative risk,” analyzes the risk associated to the exposed group over the risk associated to the unexposed group. The rate at which the exposed group experiences disease is the numerator and the rate at which the unexposed group experiences disease is in the denominator.
What is the complete formula for calculating Relative Risk (RR)?
RR = [a/(a+b)] / [c/(c+d)]
[a/(a+b)] = the risk in the exposed group
(a+b) = all the members in the exposed group
a = diseased cases in exposed group
b = non-diseased cases in the exposed group
[c/(c+d)] = the risk in the unexposed group
(c+d) = all the members in the unexposed group
c = diseased cases in unexposed groups
d = non-diseased cases in the unexposed group
What is the significance of ( 1 - RR ) ?
This is the relative risk reduction and provides the proportion of risk reduction attributable to the intervention/treatment (ART) as compared to a control (ARC).
For example, if 2% of patients who receive a flu shot develop the flu, while 8% of unvaccinated patients develop the flu, then RR = 2/8 = 0.25, and RRR = 0.75.
When you take the past exposures and compare those exposures to those who were not exposed, this is a(n) ________ study that uses a(n) ________ to make the comparison.
When you take the past exposures and compare those exposures to those who were not exposed, this is a(n) case-control study that uses a(n) odds ratio to make the comparison.
Odds ratio (OR) is the measure of association used in case-control studies. It compares the odds of exposure in cases to the odds of exposure in controls. A case-control study is retrospective in nature. It starts with individuals who have a specific outcome (cases) and those who do not (controls). The study then looks back in time to compare exposure histories between the two groups. Case-control studies are particularly useful for studying rare diseases or outcomes because they focus on individuals who already have the outcome of interest. The odds ratio in these studies provides an estimate of the relative risk, especially when the outcome is rare.
What is the formula for calculating Odds Ratio (OR)?
OR = (a/c) / (b/d) = (ad) / (bc), using the 2×2 table structure.
a = Exposed and Diseased
c = Unexposed and Diseased
b = Exposed and Non-Diseased
d = Unexposed and Non-Diseased
What is the key difference between cohort studies and case-control studies in terms of risk measures?
Cohort studies measure incidence prospectively and calculates the relative risk. The relative risk compares the probability of developing an outcome between two groups over a certain period of time. It implies a prospective study design because the patients are followed over time to see whether or not they develop an outcome. Relative risk determines within certain period of time, how many times are exposed people likely to develop a disease compared to those who have remianed unexposed.
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Case-control studies makes use of retrospective data and calculates the odds ratio. The odds ratio compares the chance of exposure to a particular risk factor in cases and controls. Cases have developed the disease and controls remain disease free. “Risk,” in it’s purest sense is not calculated, instead infered indirectly in case-control studies, because the anaylsis is retrospective. Odds ratio answers how many times are diseased people more likely to have been exposed to a particular factor in comparion to those who have not developed disease.
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Both relative risk and odds ratio are measured on a scale from 0 to infinity. The value of 1.0 indicates no difference between the two groups being compared.
What is the ‘rare disease assumption’?
The ‘rare disease assumption’ states that when a disease is rare, the Odds Ratio approximates the Relative Risk.
What does a Relative Risk (RR) or Odds Ratio (OR) of 1 indicate?
An RR or OR of 1 indicates no difference in risk between the exposed and unexposed groups.
A Relative Risk (RR) > 1 would mean that …
Indicates that the exposure is associated with an increased risk of the outcome. For example, an RR of 2.0 means the exposed group is twice as likely to develop the outcome compared to the unexposed group.
A Relative Risk (RR) < 1 would mean that …
Indicates that the exposure is associated with a reduced risk of the outcome. An RR of 0.5 means the exposed group has half the risk of developing the outcome compared to the unexposed group.
An Odds Ratio (OR) = 1 would indicate that …
Indicates that there is no association between the exposure and the outcome. The odds of the outcome are the same in both the exposed and unexposed groups.
How do you interpret an Odds Ratio (OR) > 1?
An OR > 1 suggests that exposure is associated with higher odds of the outcome occurring. For example, an OR of 2.0 means the odds of the outcome are twice as high in the exposed group compared to the unexposed group.
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Odds Ratio (OR) < 1 would indicate that …
This indicates that the exposure is associated with lower odds of the outcome. For example, an OR of 0.5 means the odds of the outcome are half as likely in the exposed group compared to the unexposed group.
