Questions and Answers
What is risk?
An estimation of the likelihood of an outcome in a specified population, over a specified time period.
Which of the following measures can be used to quantify excess risk? (Select all that apply)
Risk = Cumulative incidence in the population (Ic) = # of events in a specified time frame / _____
in population at risk
What does RD = 0 indicate?
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What can be used to calculate excess risk besides incidence measures?
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Risk Ratio is equal to the incidence in exposed divided by the incidence in unexposed.
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Which of the following formulas represents Attributable Risk (AR)?
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What is the relationship between Risk Difference and Attributable Risk?
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What does the Population Attributable Risk Percent (PAR%) indicate?
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When is the Odds Ratio a good estimate of the Risk Ratio?
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Vaccine Efficacy = Iunvacc. – Ivacc. x _____
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Attributable Risk Percentage can only be used for established causal associations.
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What does the term 'exposed' refer to in terms of cases above?
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Study Notes
Measures of Excess Risk
- Risk is an estimation of the likelihood of an outcome in a specified population over a specified time period.
- It can be calculated as the cumulative incidence in the population (Ic) or the incidence rate.
- Epidemiologists often use "risk" to refer to both cumulative incidence and incidence rate measures.
Cumulative Incidence (Ic)
- Calculated as the number of events (e.g., lung cancer cases) in a specified time frame divided by the number of people at risk in the population.
- Assumes the population at risk remains constant.
Incidence Rate
- Calculated as the number of events (e.g., lung cancer cases) divided by the total person-time of follow-up.
- Can be used to calculate excess risk measures.
Excess Risk Measures
- Used to quantify the association between an exposure and an outcome.
- Excess risk measures include:
- Risk Difference (RD)
- Risk Ratio (RR)
- Attributable Risk (AR)
- Attributable Risk Percent (AR%)
- Population Attributable Risk (PAR)
- Population Attributable Risk Percent (PAR%)
Risk Difference (RD)
- Calculated as the incidence in the exposed group minus the incidence in the unexposed group (Ie - Io).
- Interpreted as the excess risk of the outcome due to exposure.
Risk Ratio (RR)
- Calculated as the incidence rate in the exposed group divided by the incidence rate in the unexposed group (Ie / Io).
- Interpreted as the strength of the association between exposure and outcome.
Attributable Risk (AR)
- Calculated as the incidence in the exposed group minus the incidence in the unexposed group (Ie - Io).
- Interpreted as the amount of disease incidence among the exposed that is due to the exposure.
Attributable Risk Percent (AR%)
- Calculated as the AR divided by the incidence in the exposed group multiplied by 100.
- Interpreted as the percentage of disease incidence among the exposed that is due to the exposure.
Population Attributable Risk (PAR)
- Calculated as the incidence in the total population minus the incidence in the unexposed group (It - Io).
- Interpreted as the amount of disease incidence in the total population that is due to the exposure.
Population Attributable Risk Percent (PAR%)
- Calculated as the PAR divided by the incidence in the total population multiplied by 100.
- Interpreted as the percentage of disease incidence in the total population that is due to the exposure.
Odds Ratio (OR)
- Calculated as the ratio of the odds of exposure among cases to the odds of exposure among controls.
- Can be used to estimate the RR and AR% in case-control studies.
Estimating AR, AR%, and PAR% from Case-Control Studies
- AR% can be estimated even if Ie and Io are unknown, but only if the OR is a good approximation of the RR.
- PAR% can be estimated when OR is a good approximation of RR, even if Ie and Io are not known.### Estimating PAR% from Case Control Study
- A case control study on lung cancer has 140 cases with 2+ meals cooked/day and 10 cases with 1 meal cooked/day, while the control group has 200 individuals with 2+ meals cooked/day and 50 individuals with 1 meal cooked/day.
Measures of Excess Risk
- Risk ratio (RR) is used to determine if exposure (E) is related to disease (D).
- Odds ratio (OR) is used to determine if exposure (E) is related to disease (D).
- Risk difference (RD) is used to determine if exposure (E) is related to disease (D).
- Attributable risk to the exposed (AR) measures the percentage of the exposed who develop the disease due to exposure.
- Attributable risk percentage (AR%) measures the percentage of cases among the exposed that are due to exposure.
Population Attributable Risk
- Population attributable risk (PAR) measures the percentage of the population that develops the disease due to exposure.
- Population attributable risk percentage (PAR%) measures the percentage of cases in the population that are due to exposure.
Special Excess Risk Measures
- Mortality rate ratio is used to compare mortality rates between groups.
- Mortality rate ratio is calculated as the number of fatal cases divided by the total person-time at risk.
Example of Mortality Rate Ratio
- The age-adjusted heart disease mortality rates in the US from 2000-2014 show a mortality rate ratio of 1.60, indicating that the incidence rate of death from heart disease in males was 60% higher than in females of a similar age.
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Description
Learn to quantify measures of excess risk using incidence rates and estimates, including risk difference, risk ratio, attributable risk, and population attributable risk.
