# Glossary of Evidence-Based Medicine and Statistical Terms

Term   Abbreviation Definition

Sensitivity   Sn Percentage of patients with disease who have a positive test for the disease in question

Specificity   Sp Percentage of patients without disease who have a negative test for the disease in question

Predictive value (positive and negative)   PV+
PV-
Percentage of patients with a positive or negative test for a disease who do or do not have the disease in question

Pretest
probability
Probability of disease before a test is performed

Post-test
probability
Probability of disease after a test is performed

Likelihood ratio   LR LR >1 indicates an increased likelihood of disease, LR <1 indicates a decreased likelihood of disease. The most helpful tests generally have a ratio of less than 0.2 or greater than 5.

Relative risk reduction   RRR The percentage difference in risk or outcomes between treatment and control groups. Example: if mortality is 30 percent in controls and 20 percent with treatment, RRR is (30-20)/30 = 33 percent.

Absolute risk reduction   ARR The arithmetic difference in risk or outcomes between treatment and control groups. Example: if mortality is 30 percent in controls and 20 percent with treatment, ARR is 30-20=10 percent.

Number needed to treat   NNT The number of patients who need to receive an intervention instead of the alternative in order for one additional patient to benefit. The NNT is calculated as: 1/ARR. Example: if the ARR is 4 percent, the NNT = 1/4 percent = 1/0.04 = 25.

Number needed to harm   NNH The number of patients who need to receive an intervention instead of the alternative in order for one additional patient to experience an adverse event.

95 percent confidence interval   95% CI An estimate of certainty. It is 95% certain that the true value lies within the given range. A narrow CI is good. A CI that spans 1.0 calls into question the validity of the result.

Systematic review     A type of review article that uses explicit methods to comprehensively analyze and qualitatively synthesize information from multiple studies

Meta-analysis     A type of systematic review that uses rigorous statistical methods to quantitatively synthesize the results of multiple similar studies