Confidence interval
The range of values consistent with the trial's data, usually given at 95%. A hazard ratio of 0.70 with an interval of 0.55 to 0.89 means the true effect is probably somewhere in that range; if the range crossed 1.0 the result would not be significant.
Because a trial studies a sample rather than every patient in the world, its estimate carries uncertainty, and the confidence interval expresses it: a narrow interval comes from a large trial with many events, a wide one from a small trial. For a hazard ratio, an interval that excludes 1.0 corresponds to a p-value below 0.05; for a difference in months or percentage points, the interval must exclude zero. Reading the interval rather than just the point estimate shows how large or small the true benefit could plausibly be, and an interval whose upper end is close to 1.0 signals a fragile result even if it is technically significant.