You are right — much of the data is missing, thanks for pointing that out.
In the spirit of this article — encouraging the common person to take data into their own hands — I wasn’t as strict on analysis as I would have been with a more formal project.

It is true that with so much missing data, in a formal project it would be irresponsible to take analysis, but since the results match what has generally been found by more statistically rigorous studies to a reasonable degree, I think it’s fair to say that the values we know are representative enough for the purposes of this article.

I’ll make sure, however, to put a disclaimer about the high number of missing data. It should show up as soon as the editors approve it.

Thanks!