Confidence intervals

Most pharmacists will be aware of p values in terms of whether or not an answer is significant (in a statistical sense). However, the use of p values is now largely being replaced by the concept of CIs.

The CI, usually set at 95% limits (although others may be used), is a way of describing the confidence that can be placed on the ‘result’. By describing the CI the range of the result can be clearly seen, in this case with a confidence of 95% that the ‘result’ is correct.

CIs figure largely in many systematic reviews, meta-analyses and for describing numbers-needed-to-treats (NNTs). For example, the NNT for a particular drug in moderate or severe postoperative pain may be 3.6 (95% CI 3.0 to 4.4). While it is tempting to concentrate on the single figure of 3.6, in reality the answer lies somewhere between 3.0 and 4.4.

Systematic review or meta-analysis?

Generally, the term ‘systematic review’ refers to the publication of a review carried out systematically in a way that minimises bias. Within the review there may be an analysis known as a meta-analysis, presenting data graphically often in the form of a Forest plot (see below). In North America the term ‘meta-analysis’ is sometimes used to describe the review.

Forest plots

Some studies touch or even cross over the vertical line of no difference. These studies are not statistically significant in terms of a …