Monday, March 11, 2013

The Measurement of Babies Redux

A followup to my earlier post, and also apropos some other recent discussion regarding null hypotheses on paper distributions and the general scientific method: in my earlier post on measuring babies, I stated that we had observed Harriet being taller than a 30-inch carpentry level, and then wandered off into a discussion of height and weight distributions with respect to infant age.  At her six-month checkup appointment, not all that long after that post, her pediatrician found a much less startling height, somewhere around 28 inches (I can't remember exactly).

I have no doubt that the doctor got the right number---their measurement system is actually fairly ingeniously simple.  You simply lay the baby down on the disposable paper that gets pulled out to cover the examination table, mark a line tangent to the feet, and then mark up on top of the head.  With good hands and a compliant baby, getting those two marks right is easy.  Then measure between the marks, and the length of any normally growing baby is enough relative to likely sideways displacement that any distortion from angle is likely to be quite small (my quick-and-dirty estimate is that at Harriet's height, a 1-inch sideways displacement should give less than 1% error).  It's imperfect, but pretty damned good.

So, what about our earlier measurement of 30 inches?  As with most surprising experimental results, it boils down to simple experimental error.  Not quite so dramatic as accidentally finding particles moving faster than the speed of light, but then we're dealing with a much smaller scale and more poorly controlled experiment.

What could have caused it?  Remember, Harriet was playing with the level, so we weren't exactly dealing with a stable instrument.  I certainly didn't look to see whether the level was actually level, so it may have been leaning somewhat.  I may also have suffered from some degree of an optical illusion since I was looking downward, with first Harriet and then the level further from me.  I may have counted some of her fluffy hair without realizing it.  She was being partially supported by me, as she worked on her great (and slightly premature) ambition of standing, so she may well have been stretching upward in some way.  At the end of the day though, if an error doesn't persist, it's probably not worth trying to investigate its causes, since they are likely to be transient and, frankly, boring.

One of the most important lessons of science, I think, is embedded in this experience: most things that appear extremely unusual actually are not.  Instead, most compellingly unusual things are the result of some combination of happenstance and circumstance, and our cognitive bias for noticing unusual things plucks them out of the background noise and throws them into stark salience.  For example, I can remember quite clearly the circumstances of Harriet playing with the level, but can't remember just what the doctor actually measured.  It's not a mistake to pay attention to unusual-seeming things: certainly, it has been evolutionarily adaptive for our species, and still is.  But it's equally important to remember that our unusualness detectors are tuned up so high that they give us constant false positives, and that is because those few circumstances where there really is something there make it all worthwhile.  Sometimes it saves us from a stalking leopard or drunk driver. Other times, it is as in the quote attributed to Asimov: "The most exciting phrase to hear in science, the one that heralds new discoveries, is not 'Eureka!' (I found it!) but 'That's funny ...'"  It's just that finding one "That's funny..." requires going through a rather large number of places where it turns out not to be after all.

So, an interesting lesson in the banality of experimental error and the importance of proper metrology (which appears to be one of my current favorite scientific concepts, thanks in no little part to my ongoing work in synthetic biology).  With regards to the measurement of babies, however, in the end the same judgement applies: we have a long trim baby (though she's coming more into standardized proportion, which of course doesn't mean a damned thing).

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