Scary tidbit from my sociology textbook:

[Brigid]

"Today, 87 percent of people in the United States can be identified given only their gender, ZIP code, and date of birth."

[cthulu2016]

1. Well, let's see...

DOB = 365 x (call-it) 75 years... about 27,000 unque DOB per zip code

Gender roughly doubles the refinement of DOB, so call it 50,000 DOB/Gender combinations per zipcode

About 44,000 zipcodes.

So about 2.2 Billion DOB/Gender/ZIP combinations.

Yup, even accounting for the fact that not all zipcodes have people in them (soe are business or government reserved zips) it seems more than plausible

[salvorhardin]

2. Latyana Sweeney showed that in 2000

The Massachusetts Group Insurance Commission had a bright idea back in the mid-1990s—it decided to release "anonymized" data on state employees that showed every single hospital visit. The goal was to help researchers, and the state spent time removing all obvious identifiers such as name, address, and Social Security number. But a graduate student in computer science saw a chance to make a point about the limits of anonymization.

Latanya Sweeney requested a copy of the data and went to work on her "reidentification" quest. It didn't prove difficult.

...

But it was only an early mile marker in Sweeney's career; in 2000, she showed that 87 percent of all Americans could be uniquely identified using only three bits of information: ZIP code, birthdate, and sex.

Link: http://arstechnica.com/tech-policy/2009/09/your-secrets-live-online-in-databases-of-ruin

It's problematic because medical and other researchers depend on anonymized data, and that's the sort of thing we want to be open and free, but if it's very hard to actually anonymize data, then what do you do?

[Speck Tater]

3. Back in the 1800's

It was common to address a letter with first and last name and name of the city.*
If you added date of birth, name, city, and date of birth would have been enough to identify close to 100% of the U.S. population before 1900, and maybe even till 1920 or 1930, at a guess.

*People even used to play games with mail addresses, like:

HILL
-----
JAMES
-------
MASS

(James Underhill Andover Mass. which was an adequate and acceptable postal address back in those days.)

[WillowTree]

4. I dated a fella in college who was from a small town in Wisconsin.

He addressed mail to his parents with just their last name and the ZIP Code.

[Speck Tater]

5. When I was a kid our small town had four-digit phone numbers.

And if you wanted to call outside of town you had to call the operator.