[arjunrc]

I love this inference from the data.

An examination of the histogram shows significant seasonal variations. The months July - October show higher than expected births and March - May show the most significant decline in births. Perhaps the most reasonable explanation is that conceptions are up in the months of October through January and down in June through August. You be the judge.

Wonder if HN has an explanation as to why :)

[jedberg]

This data is from North America, and it gets cold and people stay in more October to January.

It would be interesting to see if the trend is reversed in the Southern Hemisphere, although you'd be hard pressed to find anywhere that gets as cold in the winter as North America does that has a similar wealth as North America.

[molmalo]

For similar latitudes, maybe New Zeland, and some places in the Argentine Patagonia.

[bostonpete]

That statement jumped out at me as ridiculous. Of course conceptions are up in those months if there is increased frequency of births 9 months later. That's not an "reasonable explanation" of the data so much as a restatement of it...

[aaronharnly]

Just to play devil's advocate -- not every conception results in birth, and it's not implausible that the P(birth|conception) might vary over the year. For example, if a first trimester in the winter were more likely to result in miscarriage; or if more unwanted conceptions resulting in abortion happened in spring (??).

But, yeah, I agree :)

[bostonpete]

Hmm, good point. Whether it's a reasonable assumption or not, I had unconsciously assumed the ratio of births to conceptions would be constant. You get an A for logic, I'll give myself a C... :-)