Forensic evidence that has helped convict thousands of defendants for nearly a century is often the product of shoddy scientific practices that should be upgraded and standardized, according to accounts of a draft report by the nation’s pre-eminent scientific research group.

The report by the National Academy of Sciences is to be released this month. People who have seen it say it is a sweeping critique of many forensic methods that the police and prosecutors rely on, including fingerprinting, firearms identification and analysis of bite marks, blood spatter, hair and handwriting.

The report says such analyses are often handled by poorly trained technicians who then exaggerate the accuracy of their methods in court. It concludes that Congress should create a federal agency to guarantee the independence of the field, which has been dominated by law enforcement agencies, say forensic professionals, scholars and scientists who have seen review copies of the study. Early reviewers said the report was still subject to change.

The result of a two-year review, the report follows a series of widely publicized crime laboratory failures, including the case of Brandon Mayfield, a lawyer from Portland, Ore., and Muslim convert who was wrongly arrested in the 2004 terrorist train bombing in Madrid that killed 191 people and wounded 2,000.