Final answer:
The MIT study on 1.1 million shoppers' credit-card transactions demonstrated the possibility of re-identifying individuals from anonymized data but the exact percentage re-identified was not specified in the provided information.
Step-by-step explanation:
In a study conducted by MIT involving 1.1 million shoppers' credit-card transactions, the researchers found that even when personal identifiers were removed from the transaction data, they were able to re-identify individuals with a surprising accuracy.
This study showcases the potential for de-anonymization of large datasets and raises significant concerns about privacy and data protection. Unfortunately the specific percentage of the study population whose identities could be determined was not provided in the references available. This calls attention to the broader issue of data privacy and the challenge of anonymizing large datasets in a world where individuals frequently leave digital trails through their financial transactions.