One Long Island hospital got an F and three scored A’s, down from seven last year, in the spring ranking of patient safety by a national health quality group.

The 250-bed John T. Mather Memorial Hospital in Port Jefferson received its ninth consecutive A, The Leapfrog Group said.

“We’re always trying to be perfect,” said Joan Faro, chief medical officer. The hospital tries to anticipate possible problems and build systems to prevent them, she said.

Leapfrog, a nonprofit founded by large companies, grades hospitals to improve their quality, examining errors, injuries, infections and drug mix-ups.

Northwell Health in Huntington and St. Francis Hospital in Roslyn also won A’s, in the twice-yearly report issued on Monday.

Terry Lynam, Northwell Health spokesman, said: “The public can be assured that if there are any issues identified, we are already aware of it and are working to resolve it. We do this routinely.”

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Stony Brook University Hospital got an F; its previous ratings ranged from A to C.

Stony Brook pinned the failing grade on a “misinterpretation” of survey instructions for nonclinical data. “Some operational and systems measures were given ratings that do not accurately reflect our current practices,” the hospital said in a statement.

“In the clinical outcomes portion of the survey, our results have continued to improve and we are confident that we provide safe, quality care,” Stony Brook said.

Leapfrog which launched the scores in 2012, says there is a 50 percent higher risk of avoidable death in D and F hospitals than in A hospitals.

In addition to its own survey, Leapfrog said it draws data from the Agency for Healthcare Research and Quality, the Centers for Disease Control and Prevention, the Centers for Medicare and Medicaid Services, and the American Hospital Association’s Annual Survey and Health Information Technology Supplement.

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Its latest ratings of about 2,500 hospitals, including 23 on Long Island, have new measures for such afflictions as MRSA, a staph infection that resists many antibiotics, a Leapfrog spokeswoman said.