Lumpers and splitters: different approaches to understanding variations research.

The latest work of John Wennberg and colleagues represents an important advance over earlier work by targeting different domains of clinical care at a more specific level. The findings have clear implications for those interested in constraining Medicare costs and may even point a way forward for lasting reform of purchasing practices in both the public and the private sectors. Nonetheless, the latest findings and proposals leave unresolved important research and policy questions, including those involving the basic relationship between spending and outcomes and payment reforms that would promote care of more uniform quality.