Dr Molette's ducks were all treated similarly, and their livers were of more or less the same size (certainly, the fat-shedding ones were not systematically heavier).
But instead of cooking the whole things, Dr Molette removed a 200-gram sample from each liver (an average liver weighed 550 grams) and put the rest in cold storage.
Dr Molette and her colleagues raised 150 male mule ducks (a cross between Pekin and Muscovy ducks that is often used to produce foie gras) for 13 weeks in standard poultry-house conditions and then transferred them into individual enclosures.