Medical work is increasingly being subjected to objective assessment as those who pay for it seek to grasp the quality of that work and how best to improve it. While objective measures have a role in the assessment of health care, I argue that this role is currently overestimated and that no human practice such as medicine can be fully comprehended by objective assessment. I suggest that the character of practices, in which formalizations are combined with judgment, requires that valid assessment involve the perspective of the skilled practitioner. Relying exclusively on objective measures in assessing health care will not only distort our assessments of it but lead to damage as the incentives of health care workers are directed away from the important aspects of their work that are not captured by objective measures. © Springer Science+Business Media B.V. 2008.