The studies, published in Wednesday's Journal of the American Medical Association, cast doubt on previous research recommending people seek out a hospital that performs a high number of the procedure they will undergo.
The authors of the heart surgery study said that much of the past research looking at the association between hospital volume and performance did not adequately account for the different types of patients at different hospitals.
After analyzing the results of 267,089 of the heart surgeries at 439 hospitals in 2000 and 2001 and adjusting for risk, the study found that for every 100 additional procedures, the likelihood of dying from the operation fell only 0.7 percent.
The researchers concluded that hospital volume had "generally poor predictive accuracy" when used to identify hospitals with lower mortality rates.
The Leapfrog Group, an organization founded by businesses to reduce medical errors, had set a hospital standard of 450 of these procedures per year. An editorial that accompanied the heart surgery study called that standard "unreasonable and impractical."
In an interview with the Online NewsHour, Leapfrog Group's communications director Claire Turner said that the group was not surprised by the outcomes of the JAMA studies and preferred using risk-adjusted data instead of hospital volumes. She added that rigorous data is not always available and that information on hospital volumes has been "proven to be better than no information at all."
The second study examined the mortality rates for premature infants born in 332 U.S. hospitals from 1995 to 2000 and found that there was some association between the number of babies treated and their survival rate. Yet study author Jeanette Rogowski of the RAND Corp. said, "The number of cases treated by an individual hospital was not a very good predictor of which hospitals would have the best results."
"We found there is no factor better in predicting which hospitals would have low mortality rates for these small babies than their actual experience in the preceding years," Rogowski said.
The author's analysis found that moving babies out of the hospitals with the highest past mortality rates among those studied would save ten times more lives than moving them out of the hospitals that have treated the fewest premature infants.