1 For each question, the participants indicated a self-estimated certainty, graded from 0 to 3: 0=very buy Letrozole uncertain, and would search for help; 1=relatively uncertain, and would probably search for help; 2=relatively certain, and would probably not search for help;

and 3=very certain, and would not search for help. The questionnaires used are enclosed as online supplementary additional file 1. Risk of error Risk of error was estimated by combining knowledge and certainty for each question rated on a scale from 1 to 3, devised for the study. Correct answer combined with relatively or high certainty was regarded as a low risk of error (score=1), any answer combined with relatively or very low certainty was regarded as a moderate risk of error (score=2), and being very or relatively certain that an incorrect answer was correct was regarded as a high risk of error (score=3). Course evaluation After the course, the nurses recorded their assessment of the level of difficulty of the course related to their own prior knowledge (1=very difficult, 2=relatively difficult, 3=relatively

easy, 4=very easy); and course satisfaction (1=very unsatisfied, 2=relatively unsatisfied, 3=relatively satisfied, 4=very satisfied). An evaluation of the usefulness of the specific course in drug dose calculations in daily work as a nurse was rated from 1=very small, 2=relatively small, 3=relatively large to 4=very large. Ethical considerations All participants gave written informed consent. The tests were performed de-identified. A list connecting the study participant number to the names was kept until after the retest, in case any of the participants had forgotten their number. To protect the participants

from any consequences because of the test, the data were made anonymous before the analysis. Even if the study might uncover that individuals showed a high risk of medication errors due to lacking calculation skills, it was considered ethically justifiable not to be able to expose their identity to their employer. Data analysis The analysis was performed with intention-to-treat analyses. In addition, a per protocol analysis was performed for the main results. Depending Drug_discovery on data distribution, comparisons between groups were analysed with a χ2 or Fisher’s exact test, a t test or Mann-Whitney U test, analysis of variance, Friedman, and Pearson or Spearman tests for correlations, and a Wilcoxon signed-rank test for paired comparisons before and after the course. All variables possibly associated with the learning outcome and change in risk of error were entered in linear regression analyses to identify independent predictors.18 Two-tailed significance tests were used, and a p value <0.05 was considered statistically significant. The protocol contained instructions for handling missing data. Unanswered questions were scored as ‘incorrect answer’, and unanswered certainty scores as ‘very uncertain’.