William M. Briggs

Statistician to the Stars!

Climate Change Causing Short Peruvians!

The authors' Figure 1.

The authors’ Figure 1.

Who remembers those crank scientists who wanted to genetically engineer human beings so that they would be sprier and narrower and thus have smaller “carbon footprints”? We don’t need ’em!

At least, not according to the peer-reviewed paper “El Niño adversely affected childhood stature and lean mass in northern Peru” in the new and exciting journal Climate Change Responses—it looks like it will prove fertile pickings for posts about bad statistics—by Heather Danysh and a handful of others.

Before we begin, let us warn ourselves against height triumphalism. Picking on short people is wrong and hurtful. We would never use terms like “height deficits” like the cruel authors of the paper do.

To the science! The hypothesis: “Due to severe food shortages and increased incidence of infectious diseases during El Niño, we hypothesized that children born in northern Peru during and after the 1997–1998 El Niño may be shorter for their age and sex than children born in other years.”

(Incidentally, there are more typos per paragraph in this article than in any post of mine. May give some indication of this journal’s quality.)

The authors were keen on height-for-age, or HAZ, and bioelectrical impedance. Impedance is used to guess the amount of lean mass versus water, a method which requires some form of calibration and which always gives an answer which is not certain and thus which should always be accompanied by some measure of uncertainty. This, of course, did not happen here. Keen readers should delve into the paper for their description of a “flood likelihood score” (instead of measuring whether kids lived through a flood, they calculated an index to guess: the Epidemiologist fallacy lurks).

Kids born between 1991 and 2001 were measured. The authors used a “linear mixed model” to explain “height in later childhood” on these “variables”: years between birth date and January 1991, and “the number of years between the child’s birth date and the onset of El Niño for the each child born after the onset of El Niño” in 1997.

Now 1997-1998 was, according to the Oceanic Niño Index (ONI), a strong event. But moderate events were seen in 1991-92, and 1994-95. And 1995-96 was classed as a weak La Niña event, and 1998-2000 moderate to strong La Niña.

So we have a mixture of signals here. What about dose-response? Did kids get taller during La Niñas?

The authors say, “All regression models were adjusted for sex, socioeconomic status (SES) index, and likelihood of living in a flood-prone household, and accounted for clustering using a random intercept by village.” Regression is always abused like this: regression does not prove causality. Say, does that SES index have uncertainty? Skip it.

The top of the post shows the author’s Figure 1. Does it look to you like that 1997-98 event shrunk kids (actually, the HAZ index)? Looks to be like the “rapid urbanization” the authors noted in their study village since 1991 might account for the generally increasing heights, no? And, hey, isn’t this a mean index with no indication of spread (not every kid had the same HAZ), i.e. a graph which is uncertain presented and used in analyses as if it were certain, a move which guarantees over-certainty? I’m just asking.

Actually, the authors’ model acknowledges the increasing (too sure) HAZ! Read that twice. Yet a wee p-value confirmed that kids born after the 1997-1998 event were increasing in HAZ at a rate slightly smaller than kids before it.

So even if the authors’ hypothesis is right, it’s not that El Niño shrunk kids, it only slowed the observed rate of increase. The increase is still acknowledge to be there. Popular accounts like Mother Jones‘ “Another Side Effect of Climate Change and El Niño Events? Shorter Kids” are thus wildly wrong. Who could have guessed?

But there is plenty of doubt about that hypothesis. Somehow El Niño knew only to effect lean and not fat mass of the kids. Wee p-values confirmed “children born after the onset of El Niño have significantly less lean mass than what would be expected if El Niño had not occurred” (forgetting that we didn’t carry uncertainty of the impedance, SES, or HAZ measures forward). Or maybe urbanization produced fatter, better-fed kids?

Now if the authors were to properly carry the uncertainty of all their measures forward, and they properly took into account what kids actually ate, it’s almost certain the wee p-values would become un-wee, thus canceling all headlines.

Yes, the Epidemiologist Fallacy has struck again. El Niño cannot cause shrunken kids. Starvation can—and so can a host of other things. Yet none of these things were measured here. None.

It’s well past the time for anybody to take seriously these Oh-My-God correlation studies. Science should be the search for causes, not the production of goofy statistics.

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20 Comments

  1. “we hypothesized that children born in northern Peru during and after the 1997–1998 El Niño may be shorter for their age and sex than children born in other years”

    It seems to me that the “look at the data” method of science might have been able to answer that question without statistics. This is just the “define what you mean by a trend” problem all over again.

  2. I long for the days when this would have been “natural variation” much as Darwin discussed. I watched a program on the changes in beak shape and sizes in birds in the Galapagos island many years back, talking about how changes can occur quickly rather than slowly, as Darwin thought. The changes occurred due to the availability of seeds. The size and toughness of the seeds available affected the survival of the birds with various beak shapes. Now, if I were watching the same show today, it would undoubtedly be climate change causing either the beak “deformations” or the seed availability, not natural selection. I miss science…….

  3. @James,

    “Oh-My-God correlation studies” almost always involve the define what you mean by a trend problem.

  4. Just one comment (after reading the link to the Epidemiologist Fallacy post). How’s the book coming along? 🙂

  5. What about the alarming increase of Plumbers Syndrome (droopy pants) in American youths? And the expanding incidence of Dunlop Disease (wherein the belly done lop over the belt) in American males of mature vintage? Both maladies are very obviously correlated with Climate Disruption. Perhaps even the causes of it?

  6. Uncle Mike: …Both maladies are very obviously correlated with Climate Disruption. Perhaps even the causes of it?…

    Are you saying Climate Disruption causes “Plumbers Syndrome” and “Dunlop Disease” or …

    Are you saying “Plumbers Syndrome” and “Dunlop Disease” causes Climate Disruption (the IPCC will want a full report)

    (My wife makes me wear ‘spenders to prevent the one and walking the dog -Bella Moose- helps mitigate the other)

  7. Global Warming,er, Climate Change is a powerful thing.
    http://www.imdb.com/name/nm5956253/?ref_=ttfc_fc_cl_t30

    Randy Newman is apparently not in favor of GW/CC.

  8. Briggs

    December 1, 2014 at 2:27 pm

    DAV,

    Better version:

    https://www.youtube.com/watch?v=1NvgLkuEtkA

    Uncle Mike!

    Nice to see you.

  9. I assumed this was a practical joke study until I checked the paper myself. Your tax dollars at work.

  10. Yeah, DAV, Randy Newman immediately came to my mind too.

    El Niño was named by the Peruvian anchovy fishermen who noticed the fish disappeared when the waters turned abnormally warm around Christmas time every few years. Not good when you catch no fish — your children go hungry.

  11. Nobody’s kids go hungry… They are not as wealthy as Americans or Canadians or Australians, but they don’t carry spears and wear grass skirts either. (Shakes head.)

  12. (Incidentally, there are more typos per paragraph in this article than in any post of mine. May give some indication of this journal’s quality.)

    Obviously they have more enemies than you have and they struck with a vengeance.

  13. Will, you need to tune up your hyperbole detector. 😉

  14. What I get from that figure is that there was an unusually warm winter in 1997 (assuming the labels apply to January) and that this was co-incident with a high SST (whatever that is) and heavy summer rainfall. And then things went back to mostly “usual”.

    I’d also be interested in seeing the “p-value” for height-for-age and maximum temperature from 1997 onwards. Looks to me like max temperature is roughly falling while height is roughly increasing. Obviously, taller people cause lower temperatures, and we should feed young kids growth hormones to suppress global warming.

  15. SST is sea surface temperature.

    I like your idea of feeding kids growth hormones to suppress global warming. Less frightening than ideas about geoengineering, giant mirrors, etc. Except maybe to the kids.

  16. Before everyone concludes that the people who did this study are nitwits, consider that they managed to dig around and find some rural villages near a major tourist spot, where after a hard day’s researching, they could hit the local
    liqueur and night life.

    http://www.vivatv.pe/fotos/nacionales/TUMBES_14092011.jpg

    http://tipsfamilia.com/wp-content/uploads/2011/03/tumbes1.jpg

  17. Will: That makes them clever, not smart.

  18. They’re getting shorter?

    JMJ

  19. Sander van der Wal

    December 2, 2014 at 2:27 am

    Society gets the scientists</bold it deserves.

  20. OK use of the Mark 1 eyeball suggests both that Peruvians are taller at the end of the study period than the beginning; and that the rate of increase is higher (It’s negative at the start and positive at the end…) suggesting that the hypothesis mooted is 100% Alpaca dung… Am I missing something?

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