How to Calculate a Case-Fatality Ratio

Epidemiology, the study of disease transmission, uses case-fatality ratios to analyze the lethality of infectious disease. This data can help identify how dangerous a particular pathogen is, and public health bodies can then focus resources appropriately to the risk. Case-fatality ratio is also known as case-fatality rate, or in non-epidemiological fields, as lethality.

Things You'll Need

• Public health records
• Calculator

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Instructions

• 1

  Focus on either the population as a whole or on one particular group of people, such as people aged under eighteen. Assigning specific case-fatality ratios to specific groups gives you a clearer picture of who is most at risk from the disease. In a case-fatality ratio for the entire population, details are lost.

• 2

  Count the number of cases of the disease in a particular period of time. For example, if an epidemic lasts about three months, then you can choose that span of time as your parameter. The three months should begin with the first case and end when the last case first showed symptoms. Alternatively, you can opt for time spans like one year, because government health records may be easily searchable by year.

• 3

  Fix a certain period of time in which to search the fatality rate. For example, for acute diseases such as measles, the period could be within four to six weeks after the rash symptom develops. For chronic illnesses caused by disease, the time limit could be years or decades. The point of this time limit is to eliminate people who have suffered from the disease but who may not necessarily have been killed by it.

• 4

  Analyze the outcomes of each of the cases in your chosen time span. Count the number of people who died as a result of the illness.

• 5

  Divide the number of people who died by the total number of people who contracted the disease in your specified time period.

• 6

  Convert this number into a percentage by multiplying by 100. Epidemiologists always report case-fatality ratios as a percentage. For example, if doctors diagnosed 200 people as infected in one epidemic, and five of these died, then the case fatality ratio is 2.5 percent.