It was a pattern that many researchers had noticed—children in developing countries born at certain times of the year are taller for their age than others.
William Masters, a professor at the Friedman School, wanted to know more, specifically how stress in utero affects children later in life. What he and his colleagues found instead was a widespread flaw in the data that researchers have relied on for two decades to understand childhood malnutrition.
Many of the surveys used for collecting information on children’s growth in developing countries, they found, have more error in children’s birth months than their birth years. As a result, observed differences in height by season of birth may not be due to climate-related fluctuations in nutrition or infections, as previously assumed, but to the fact that children classified as born early in the calendar year are actually younger than reported, while those reported to be born late in the year are in fact older.
Correctly classifying children’s age in months is important for the accuracy of a key child health indicator, the height-for-age score. The score, a standard measure comparing a child’s height to that of healthy peers, is commonly used to indicate malnutrition and allocate global aid.
"What we found suggests there may be significant measurement error associated with much of the literature regarding factors that influence childhood development"
Then they devised an algorithm to fix the problem. “What we found suggests there may be significant measurement error associated with much of the literature regarding factors that influence childhood development,” Masters said. “Correcting for measurement error can help us see country-specific patterns and more clearly identify areas for focusing our efforts to help children.”
Tufts Now recently spoke with Masters about the discovery and the correction formula that he and his colleagues have devised. The findings were recently published in separate studies in Demography and the American Journal of Clinical Nutrition.
Tufts Now: How did you go about unraveling this phenomenon?
William Masters: We came across this finding accidentally while trying to learn more about a pattern that researchers have observed from survey data in the developing world, that children born at certain times of the year are taller for their age than others. This seasonality in attained height was thought to be caused by seasonal variations in climate, employment, and agriculture affecting the child in utero and early infancy.
My own and others’ previous research on birth season differences focused on individual countries in Africa and South Asia. We wanted to identify how richer countries are able to protect infants from seasonality in the weather, so we took height-for-age scores and reported month of birth for nearly one million children from sixty-two different countries and plotted them on a graph. The United States Agency for International Development paid for these data, so they’re in the public domain.
Was there an “aha” moment when your realized what was happening?
The puzzle jumped out at us right away. In the global data, with so many observations, we saw a straight upward line showing children born later in each calendar year to be taller. Despite different climates around the world, we saw children born in January to be the smallest, with slight increases in height for age among those born in February, March, and each successive month with the tallest children being those reported as born in December. The graph looked like a sawtooth, rising smoothly from month to month, followed by an abrupt drop in attained height from December to January.
What we saw in the global data could not be true seasonality. In previous studies with smaller sample sizes, the fluctuations by month of birth were less smooth and could be due to real fluctuations in harvests and the disease environment. This global sawtooth pattern was surely the result of measurement error, but could not be explained by any of the known problems in survey data such as heaping, in which people report age in round numbers or record births around memorable dates, or discontinuity caused by switching between measuring infants’ length lying down to measuring older children’s height when they’re standing up.
The first aha moment was realizing that the sawtooth pattern was an artifact of measurement, not real seasonality. We then stared at the data for a long time, trying different explanations. In the end, the simplest theory worked best, reproducing the sawtooth when some of the surveyed children have randomly scrambled birth months within their birth year.