Tuesday 18 January 2011

Distribution of Birth Dates

I recently wrote about several variations of the Birthday problem, here and here. Steven Colyer from "Multiplication by Infinity" Blog pointed out in a comment that in reality the distribution of birthdays is not uniform. There are months and days that are more likely than others.

I actually have a graph that I show my Stats kids each year that illustrates this. In this age of births in hospitals the number of births on holidays and weekends is reduced. My only file on this is from 1979, and I suspect the difference is even more exaggerated in more recent years.

In this graph the day of the year (1-365) is on the x-axis and the number of babies born is on y.

You can see the Sat/Sun values are about 80% of the weekdays. You can also see a rise in births in the late summer, about July to September.
Ok, if you want this one, you can find it at the chance data base which you can cut and paste to almost any statistical software.

If anyone has a more current birth by date data chart for a single year I would love to have it.

Roy Murphy (murphy@panix.com) retrieved birthdates from 480,040 insurance policy applications made between 1981 through 1994 of a Life Insurance Company.
Here is the distribution by month compared to the expected number:

Also, for those who are sure to ask...
A scientific study conducted in 1994 found that "scientific analysis of
data does not support the belief that the number of births increases
as the full moon approaches, therefore it is a myth not reality."
"Labor ward workload waxes and wanes with the lunar cycle, myth or reality?"
source: PUBMED, National Libary of Medicine, hosted by nih.gov


Mary O'Keeffe said...

In recent years, there has been a significant shift from January birthdays back to December birthdays, due to the large increase in child-related tax benefits. A baby born before midnight December 31 can entitle his parents to thousands of dollars worth of tax credits a few week's later. If the baby is born after midnight Jan 1, the parents will have to wait over a year for those tax credits. More details here:

Pat's Blog said...

Thanks.. I just looked at one of the articles linked from your post and will add a followup blog with the notes... and graphs.

Steven Colyer said...

Wow, so Crilly was mostly right. If July through October are the top months for births, may we then conclude that October through January are the top months for making love?

Wow! I always knew Autumn was my favorite season, but I had no idea WHY. I mean, there's pros and cons to every season. Why should one have precedence over the other?

Now I know why, thanks to you and Crilly. :-)

Mostly you though, because I didn't actually read that whole 4-page chapter in Crilly's book, until you raised the Birthday "problem" here at your blog, so, thanks!

So stop reading, folks! It's January, the end of the "hot" season! Time to get cookin'!


Arun said...

And the football season competes with that???

Steven Colyer said...

If you're talking American football (the rugby + soccer variant + steroids + padding - healthy kneecaps), then ... Yes!

When a young man's team's wins, that gets the juices flowing, believe that. So, given the recent success of the New York Jets, expect an uptick in the babies born in Queens and Brooklyn 9 months from now.

Not in Boston so much ... the Mighty Brady has struck out. :-)