[jasonkolb]

Your DOB and last 4-5 of your SSN matched someone else?

The reason I ask is because rules like this are often used to de-duplicate records. It's not perfect but it is useful, especially when trying to integrate data from more than one system. It's also used quite a bit in fraud detection etc. to find connections in the data.

[tracker1]

There were about 380K users in the various systems... so conflict chances were pretty high... I can't imagine what it would be like to have a name like "John Smith" or "Adam Jones" ... even more common...

[Dove]

Well, there are 366 dates of birth, and 9999 last 4 digits in SSNs, so approximately 3.7M combinations.

If you drop 380k users into 3.7M buckets, that's only ten times as many buckets as users. A lot of the buckets will be shared.