A common feature of continuous data is that a measurement used to assess the outcome of each participant is also measured at baseline, that is before interventions are administered. This gives rise to the possibility of using differences in changes from baseline (also called a change score) as the primary outcome. Review authors are advised not to focus on change from baseline unless this method of analysis was used in some of the study reports.
When addressing change from baseline, a single measurement is created for each participant, obtained either by subtracting the final measurement from the baseline measurement or by subtracting the baseline measurement from the final measurement. Analyses then proceed as for any other type of continuous outcome variable using the changes rather than the final measurements.
Commonly, studies in a review will have used a mixture of changes from baseline and final values. Some studies will report both; others will report only change scores or only final values. As explained in Chapter 9 (Section 9.4.5.2), both final values and change scores can sometimes be combined in the same analysis so this is not necessarily a problem. Authors may wish to extract data on both change from baseline and final value outcomes if the required means and standard deviations are available. A key problem associated with the choice of which analysis to use is the possibility of selective reporting of the one with the more exaggerated results, and review authors should seek evidence of whether this may be the case (see Chapter 8, Section 8.14).
A final problem with extracting information on change from baseline measures is that often baseline and final measurements will be reported for different numbers of participants due to missed visits and study withdrawals. It may be difficult to identify the subset of participants who report both baseline and final value measurements for whom change scores can be computed.