Spark
Loading...

Calculating Attack Rate, Relative Risk, Odds Ratio

In epidemiology, calculating attack rate, relative risk, and odds ratio helps scientists understand how diseases spread and what factors influence their occurrence. The attack rate measures the proportion of a population that becomes ill during a specific time, which is crucial for predicting and managing outbreaks. Relative risk compares the probability of an outcome in an exposed group to that in an unexposed group, helping determine if a factor increases or decreases disease risk. The odds ratio, often used in case-control studies, quantifies the strength of association between two events, such as exposure to a risk factor and the development of a disease.

Figures (4)

The group exposed to treatment (left) has half the risk (RR = [4/16]/[8/16] = 0.5) of an adverse outcome (dark) compared to the unexposed group (right).
Risk ratio vs odds ratio
A graph showing how the log odds ratio relates to the underlying probabilities of the outcome X occurring in two groups, denoted A and B. The log odds ratio shown here is based on the odds for the event occurring in group B relative to the odds for the event occurring in group A. Thus, when the probability of X occurring in group B is greater than the probability of X occurring in group A, the odds ratio is greater than 1, and the log odds ratio is greater than 0.
A graph showing the minimum value of the sample log odds ratio statistic that must be observed to be deemed significant at the 0.05 level, for a given sample size. The three lines correspond to different settings of the marginal probabilities in the 2×2 contingency table (the row and column marginal probabilities are equal in this graph).

Attack rate is the simplest to picture: it counts how many people in a group get sick during a specific time. You divide the number of new cases by the total number of people at risk. It tells you how much more likely the exposed group is to get sick compared to the unexposed group.

If it’s higher than 1, the exposure increases risk. If it’s lower than 1, the exposure might protect. Imagine comparing two classrooms—one with a new vaccine and one without—to see which has more flu cases.

Odds ratio is similar but used in different study types, like case-control studies. It compares the odds of being exposed in people who got sick to the odds in those who didn’t. It’s not the same as relative risk, but when diseases are rare, they’re close.

For example, if you look at people who had a disease and ask how many were exposed to a chemical, then compare to healthy people, the odds ratio tells you how much the exposure might be linked to the disease.

Key Points

  • The attack rate is the proportion of an at-risk population that contracts the disease during a specified time interval.
  • The relative risk is the ratio of the probability of an outcome in an exposed group to the probability of an outcome in an unexposed group.
  • The odds ratio is the ratio of the odds of an event occurring in one group to the odds of it occurring in another group.
  • A case control study involves selecting representative samples of cases and controls who do, and do not, have some disease, respectively.

Terms

Tap a term for a plain-language explanation.

Sources & licensing(4)