Event rate is a statistical term that describes how often an event occurs. An event can be anything such as the Chicago Cubs winning the World Series or people in a group getting a reaction from a prescription drug. The formula for event rate is the number of times the event occur divided by the possible times the event could occur.

Determine the occurrence and size of the sample you are analyzing. For instance, a researcher wants to see how many times a person gets hives from their new dishwashing detergent. To analyze the results, the researcher has 1,000 people test the product.

Determine the amount of times the occurrence happens. In our example, the researcher found 20 people broke out in hives.

Divide the number of times the occurrence happened by the total times the occurrence could happen to determine the event rate. In our example, 20 people divided by 1,000 people equals an event rate of 0.02, or 2 percent.
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