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Evidence-Based Medicine: EBM Calculations

Number Needed to Treat

The Number Needed to Treat (NNT) is the number of patients you need to treat to prevent one additional bad outcome (death, stroke, etc.). For example, if a drug has an NNT of 5, it means you have to treat 5 people with the drug to prevent one additional bad outcome.

Relative Risk 

"The ratio of the risk of an event in the experimental group compared to that of the control group (RR=EER / CER). Not to be confused with relative risk reduction." --CEBM Glossary

Odds Ratio

A statistic that quantifies the strength of association between "events" and "non-events". --CEBM Glossary

Likelihood Ratio

"The likelihood that a given test result would be expected in a patient with the target disorder compared to the likelihood that the same result would be expected in a patient without that disorder." --CEBM Glossary

Confidence Intervals

"The range around a study’s result within which we would expect the true value to lie. CIs account for the sampling error between the study population and the wider population the study is supposed to represent." --CEBM Glossary

Sensitivity and Specificity

Sensitivity is The proportion of people with disease who have a positive test. While Specificity, is the proportion of people free of a disease who have a negative test.

Additional Resources for Interpreting Statistics

EBM Calculators