[Benchmarking using different measurement instruments and the management of measurement variability]. 
In mental health care centres in the Netherlands outcome data are collected using a variety of outcome instruments.  This may have implications for the comparability of outcome results between different centres.  To discuss recent findings regarding the extent to which the eight instruments currently used in clinical practice report comparable results.  Our study is based on a combination of literature review and empirical research.  The results obtained with the eight instruments are not equivalent.  Patients symptom reductions appear larger with some instruments than with others.  The current practice of benchmarking in the Dutch mental health system would have greater validity if the number of different instruments would be reduced.  State-of-the-art calibration studies are necessary to validate the comparability of the remaining instruments.  Ideally, all mental health centres will soon use one instrument per care domain to measure treatment outcome.