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Fish samples were collected from restaurants and grocery stores across Massachusetts. Most of the testing was focused on certain species — including Atlantic cod and tuna — because they have been identified by regulators as more likely to be mislabeled.

■  Each sample included the receipt and was labeled with the date collected, restaurant or store name, and price. Samples were stored in industrial freezers in a locked room at the Globe.

■  Using a sanitized scalpel, reporters placed small pieces of fish — roughly the size of a Q-tip head — in separate ethanol-filled test tubes supplied by officials at the Biodiversity Institute of Ontario at the University of Guelph, which is home to a 26-nation consortium called the International Barcode of Life.

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■  Test tubes were sent by express mail to the Biodiversity Institute.

■  Barcode of Life researchers successfully conducted DNA analysis from most of the samples, matching them with a species in their database. Two samples were discarded because the DNA could not be read. The Globe also sent a select group of specimens to a second lab for DNA testing.

RESULTS: Of 76 fish samples collected from stores and restaurants that sold mislabeled fish in 2011, more than three-quarters of them were again mislabeled in 2012.