95% Confidence Intervals

The rates are provided with lower and upper limits of a 95% confidence interval. These are used to determine whether differences between rates are likely to be real ‘significant’ differences rather than random fluctuations. A 95% confidence interval is a range of values that is usually used to describe the uncertainty around a point estimate of a measure. In this case of mortality rates, this uncertainty arises as factors influencing the rate are subject to chance occurrences that result in random fluctuations in the rate over time and between different areas. The confidence interval is a way of conveying the stability of the rate. The smaller the confidence interval, the more stable the rate. More events lead to a smaller interval, so mortality rates from rarer causes of death have quite wide intervals and the rates are fairly unstable. Confidence intervals are presented on the charts as an I shape projecting above and below the values.