Residents in the Northern California town of Clearlake have been having two common heart procedures at five and six times the state average. Rates are also high across a wide swath of the Central Valley. These elevated numbers are partly a result of widespread health problems that show up in the population. But a new study suggests that the high rates go well beyond that, and researchers say the biggest cause may be clinical practice decisions made by doctors. The findings could have enormous implications for health costs and the quality of care. Insurers and government agencies are taking up the cause, increasingly using these so-called variation studies to pressure doctors to change their practices.