« Easy software? | Main | Appendix Slides in Beamer: Controlling frame numbers »

Birthday distribution

Many of my friends have birthdays in the next few weeks.  This fact prompted a discussion about the uniformity of birthdays.  In Outliers, Gladwell makes the case that birthdays of a group of individuals may appear skewed for subtle reasons; however such results shouldn't hold for the populace.

This question is easy to answer with a bit of Googling.  A Dartmouth professor has precisely the required data --- though only for a single year.


I used R to make a quick display of the data.

Cutting and pasting this into R produces the following output for me.

No, that isn't a data problem.  There really are two groups of birthdays.

While looking for the overall date of birth data, I discovered another file from the CDC that explains the effect.


The data in that file show many fewer births on weekends compared with weekdays.  This effect is precisely what we see in the plot, which R helps us validate.


This analysis was good enough for my own personal edification.  There is still a bit of work left to make these claims statistically valid, but that isn't my point here.


TrackBack URL for this entry:

Comments (3)

Nathan Hurst:

I'm guess this is caused by not inducing birth or performing cesareans on weekends. I guess births in a less medicalised country might show this, or alternatively find the prob born by cesarean.

C-sections could explain it. In 1978, 15% of births were via C-section (see http://www.pubmedcentral.nih.gov/pagerender.fcgi?artid=1422801&pageindex=3, Trends in cesarean section rates for the United States, 1970--78.
P J Placek and S M Taffel.)

If the data from 1970 were available (when C-sections were only 5% of births) that would help confirm.

Also, this data was only from New York State.

Ross Ihaka:

The CDC data is even more interesting. It shows that there are fewer Sunday births than Saturday ones and that there are fewer births on Monday than on other weekdays. I think this is probably explained by a deficit of early morning births the day after Saturday and Sunday.

The presence of public holidays (and even Halloween) is clearly visible.

I also saw some CDC data that indicates both C-section and vaginal births drop on the weekends.

Cool Stuff

Post a comment

(If you haven't left a comment here before, you may need to be approved by the site owner before your comment will appear. Until then, it won't appear on the entry. Thanks for waiting.)


This page contains a single entry from the blog posted on April 17, 2009 12:31 AM.

The previous post in this blog was Easy software?.

The next post in this blog is Appendix Slides in Beamer: Controlling frame numbers.

Many more can be found on the main index page or by looking through the archives.

Powered by
Movable Type 3.34