If the RR/OR/HR If the RR/OR/HR >1, and the CI does not include 1, events are significantly more likely in the treatment than the control group.If the RR, OR, or HR = 1, or the confidence interval (CI) = 1, then there is no statistically significant difference between treatment and control groups.In a person with an AR of stroke of only 0.025 without treatment, the same treatment will still produce a 20% RRR, but treatment will reduce her AR of stroke to 0.020, giving a much smaller ARR of 2.5% – 2% = 0.5%, and an NNT of 200.If a person's AR of stroke, estimated from his age and other risk factors, is 0.25 without treatment but falls to 0.20 with treatment, the ARR is 25% – 20% = 5%.But the ARR is higher and the NNT lower in people with higher absolute risks. RRR is usually constant across a range of absolute risks. ![]() RR of 0.8 means an RRR of 20% (meaning a 20% reduction in the relative risk of the specified outcome in the treatment group compared with the control group).Hazard Ratio (HR) = (risk of outcome in exposed group) / (risk of outcome in non-exposed group), occurring at a given interval of time.Odds Ratio (OR) = (odds of the event in the exposed group) / (odds of the event in the non-exposed group) = (a/b)/(c/d) = ad/bc (from 2x2 table, see below).Number Needed to Harm (NNH) = 1 / (ARt – ARc).Relative Risk Reduction (RRR) = (ARc – ARt) / ARc or RRR = 1 – RR.Relative Risk (RR) = ARt / ARc = (a/(a+b)) / (c/(c+d)) from 2x2 table, see below.Absolute Risk Reduction (ARR) = the AR of events in the control group (ARc) - the AR of events in the treatment group (ARt).Absolute Risk (AR) = the number of events (good or bad) in a treated (exposed) or control (non-exposed) group, divided by the number of people in that group. ![]() These are the definitions and relationships among various terms used to describe risk and changes in risk. Risk in statistical terms refers simply to the probability that an event will occur.
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