RLO: Confidence Intervals: The role of Chance

School of Nursing and Academic Division of Midwifery

If this trial was repeated in another group of 10 similar patients, would you expect to see exactly the same results again? Would it be surprising if, for example, 1 patient in each group had a heart attack? Would it be surprising if 2 patients in the placebo group and 3 patients in the treatment group had a heart attack?

Try changing the values in the table and look at the effect on the relative risk reduction. Note how values for the relative risk reduction changes. The true effect on relative risk reduction is somewhere within this range of values and may be larger or smaller than the effect observed in the original trial. This range, within which the true value could plausibly lie given the size of the difference actually observed, is called the confidence interval.