What's in a name?

How many people are there in the US with your exact name?

Estimated with Probability

Consider the name Stephen Flood. Even though Stephen is fairly common first name, you hear the last name Flood much less frequently.

  • According to one site, the 2000 US census found that "out of 100,000 people in the United States, [the surname] FLOOD would occur an average of 6.17 times."

  • According to another site " there were 842,384 people named Stephen out of the 335,174,936 people born between 1914-2013

Unfortunately, the US census only releases separate lists of first and last names. To simplify things a bit, lets assume (pretend) that first and last names are independent. Then

\[P(\text{Full name}) \approx P(\text{First name})*P(\text{Last name})\]

and

\[\text{Occurances of your full name} = P(\text{Full name})*\text{Total population}\]

Now we need to find out the rate at which each name occurs.

Using the information above we see that

\[P(\text{Flood}) = \dfrac{6.17}{100,000} = 0.0000617\ ( = 0.00617\%) \]

and that

\[P(\text{Stephen}) = \dfrac{842,384}{335,174,936} = .0025\ (= .25\%)\]

Putting this together, we get

\[P(\text{Stephen Flood}) \approx 0.0000617 * .0025 = 0.00000015425\ (= 0.000015425\%) \]

This sounds pretty uncommon. But the US population is fairly large. According to census.gov, the current US population is a bit below 319,000,000 people.

\[\text{Occurances of Stephen Flood in the US} \approx 0.00000015425*319,000,000 = 49.2 \]

So we might expect to see about 49 Stephen Floods in the US. We would expect to see more in the world at large, but we'd need to recalculate the relative frequency to know how many more people to expect.

Measured with Data

In fact, whitepages.com records around 100 people with the name "Stephen Flood" in the US and Canada, and there are probably many more. Our assumption that the probability of first and last names were independent was incorrect!

And indeed, a quick web search reveals a number of different 'Stephen Flood's in the US

Looking outside the US, we even find a number of of academics named "Stephen Flood"