Birthdate (month, day and year of birth), gender, and 5-digit postal code (ZIP) uniquely identifies most people in the United States. Surprised?
Perhaps at first, but then you do a quick calculation: 365 days in a year x 100 years x 2 genders = 73,000 unique combinations, and because most postal codes have fewer people, the surprise fades. Or, if you are still not convinced, there are more than 32,000 5-digit ZIP codes in the United States; so 73,000 x 32,000 is more than 2 billion possible combinations but there are only 310 million people in the United States
. In 1997, Latanya Sweeney did this kind of uniform calculation on populations reported in the U.S. Census for age groups in each postal code and summed the results to predict that at most 87 percent of the U.S. population had unique combinations [1].