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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.

http://www.dartmouth.edu/~chance/teaching_aids/data/birthday.txt

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

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

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.

http://www.cdc.gov/nchs/data/statab/t941x16.pdf

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.

dob-dayofweek

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.

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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

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