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In some studies there are background variables which are strong predictors for the outcome variable. In an observational study, the distribution of background variables often differs between the groups. Such variables may act as confounders, and cause bias in the estimated effect unless they are adjusted for in the analysis. In a randomised controlled trial, there will be no systematic differences. But it is possible to increase the statistical power, and hence the precision of the effect estimate, by adjusting for such variables (1) page 419. The effect on statistical power is studied in (2, 3), where the authors adjusted for up to four prognostic variables, and for three variables that were unrelated and were ‘noise’. Statistical power could increase substantially by including strong prognostic variables, and the power was only slightly reduced when including noise variables.

Bør man justere for bakgrunnsvariabler i en randomisert studie?