How to deal with cost differences at baseline

Van Asselt, A. D. I., Van Mastrigt, G. A. P. G., Dirksen, C. D., Arntz, A., Severens, J. L. & Kessels, A. G. H., Jun-2009, In : Pharmacoeconomics. 27, 6, p. 519-528 10 p.

Research output: Contribution to journalArticleAcademicpeer-review

BACKGROUND: To our knowledge, adjustment for baseline imbalances in costs has never been performed in trial-based cost-effectiveness analyses.

METHODS: We used data from a clinical trial performed in the Netherlands comparing two outpatient psychotherapies: schema-focused therapy (SFT) versus transference-focused psychotherapy (TFP). Costs were assessed with a cost interview. Outcome was the proportion of recovered patients measured with the Borderline Personality Disorder Severity Index (BPDSI-IV). We used three methods to adjust the costs for baseline differences: (i) mean difference adjustment, calculating total costs after baseline by adjusting the difference between groups with the difference of the mean baseline costs; (ii) delta adjustment, calculating the individual differences between patient baseline and the subsequent measurements (concerning incremental costs, this is the same as mean difference adjustment); and (iii) regression-based adjustment, adjusting total costs with a regression model, with total costs as the dependent variable and baseline costs as the independent variable.

RESULTS: Mean baseline costs were 3339 euros for SFT and 4238 euros for TFP, a mean difference of 899 euros. Total unadjusted follow-up costs were 30822 euros for SFT and 36812 euros for TFP. The fraction of recovered patients was 45% for SFT and 24% for TFP. Cost-effectiveness acceptability curves show that mean difference and delta adjustments are different from the regression-based methods.

CONCLUSIONS: Although the routine starting point of an analysis should always be an unadjusted analysis of the cost effectiveness, a baseline difference between treatment groups should be adjusted for. This should be done by reported patient characteristics or, when these are not sufficiently present, by baseline costs as a substitute. This adjustment should be carried out most preferably with a regression-based method.

Original languageEnglish
Pages (from-to)519-528
Number of pages10
Issue number6
Publication statusPublished - Jun-2009


  • article, borderline state, clinical trial, controlled clinical trial, cost effectiveness analysis, cost of illness, human, priority journal, psychotherapy, randomized controlled trial, schema focused therapy, transference focused psychotherapy

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