Absolute Risk (AR)¹

Refers to the likelihood that an individual will experience a specific outcome within a defined period. This probability can range from 0 to 1 or can be expressed as a percentage. Unlike everyday usage, where "risk" typically implies something negative, in epidemiology, it can refer to both negative events (like a heart attack) and positive events (like recovery from an illness).

Absolute Risk Increase (ARI)¹

The difference in risk between the experimental group and the control group in a clinical trial, specifically when the risk in the experimental group is higher than in the control group. It is calculated by subtracting the Absolute Risk (AR) in the control group from the AR in the experimental group. ARI provides the actual difference in risk but does not indicate the proportional increase between the two groups. To understand the proportional increase, you would use the Relative Risk (RR).

Absolute Risk Reduction (ARR)²

The decrease in risk of a particular event when comparing two groups: one receiving the treatment and the other not (the control group). It is calculated by subtracting the event rate in the treatment group from the event rate in the control group. In other words, ARR tells us how much the risk of an event is reduced by the treatment compared to no treatment.

Relative Risk/Risk Ratio (RR)²

Compares the likelihood of an event occurring in the experimental group to the likelihood of it occurring in the control group. It is calculated by dividing the event rate in the experimental group by the event rate in the control group. Essentially, RR shows how much more or less likely the event is to happen in the group receiving the treatment compared to the group that does not.

Relative Risk Reduction (RRR)²

Represents the percentage decrease in the risk of an event occurring in the treated group compared to the control group. It is calculated by subtracting the event rate in the treated group from the event rate in the control group, then dividing this difference by the event rate in the control group. RRR essentially shows how much the treatment reduces the risk of an event relative to the risk without treatment.

Odds Ratio (OR)¹

A measure used to assess the effectiveness of a treatment. It compares the odds of an event occurring in the experimental group to the odds of the same event occurring in the control group. An OR close to 1 indicates little to no difference in effect between the treatment and the control. If the OR is greater than 1, the treatment is associated with a higher likelihood of the event, while an OR less than 1 indicates a lower likelihood compared to the control. The event being measured can be either negative (like death) or positive (like survival). When events are rare, the OR is like the Relative Risk (RR), but as the frequency of events increases, the OR and RR can differ significantly.

Number Needed to Treat (NNT)²

Refers to the number of patients who must be treated to prevent one negative outcome. It is calculated as the inverse of the Absolute Risk Reduction (ARR), meaning NNT = 1/ARR.

Number Needed to Harm (NNH)²

Indicates how many patients need to receive the treatment for one additional person to experience a harmful effect compared to those receiving the control treatment. NNH is calculated as 1/ARI, where ARI stands for Absolute Risk Increase.

Likelihood Ratio (LR)¹

A measure that compares the probability of a specific test result occurring in an individual with the target condition to the probability of the same result occurring in an individual without the condition.

Positive Predictive Value (PPV)¹

The probability that a person has a disease if they receive a positive result on a diagnostic test. Unlike sensitivity, which measures the likelihood of a test correctly identifying those with the disease, PPV focuses on the likelihood that a positive test result accurately reflects the presence of the disease.