"Heavier babies do better in school" is an association claim--with an appropriate, non-causal headline--about birthweight and school achievement. The new "upshot" column in the New York Times reported this example of a study that recorded children's birthweights, and then tracked them through the 8th grade. All children in the state of Florida are tracked in this way.
Seven-pound babies appear to be healthier than six-pound babies — and to fare better in school as they age. The same goes for eight-pound babies compared with seven-pound babies, and nine-pound babies compared with eight-pound babies. Weight, of course, may partly be an indicator of broader fetal health, but it seems to be a meaningful one: The chunkier the baby, the better it does on average, all the way up to almost 10 pounds.
The graphic in the story (go check it out now. I'll wait.) shows that as birthweight increases, school acheivement scores in 3rd and 8th grade increase, too. This graphic is neither a scatterplot nor a bar graph--the two forms you learned in Chapter 8. Instead, it shows the average achievement scores for babies at each of several possible weights--connecting the dots shows how the achievement increases as weight goes up.
a) How do we know this is a correlational study? What are its variables?
b) Here's a quote from the article:
Mr. Figlio estimates that, all else equal, a 10-pound baby will score an average of 80 points higher on the 1,600-point SAT than a six-pound baby. Another way to see the pattern is to look only at top-scoring students: Among the top 5 percent of test scorers in elementary school, one in three weighed at least eight pounds at birth, compared with only one in four of all babies.
Does this quote address statistical validity? Construct validity? External validity? or Internal validity?
c) Here's a great addition. Underneath the main figure in the article, are tables of results for education, race, and age. The caption reads:
The effect of being heavier is similar across many different types of mothers.
Is this caption addressing potential moderators? potential mediators? or potential third variable problems?
d) Here's another quote from the piece:
Florida offers a window on the issue because the state tracks children from birth through college.... The authors of the new study....used the data to compare birth weight with test scores from third through eighth grades, as well as with kindergarten readiness scores. They controlled for, among other factors, the health and sex of the baby, the length of the pregnancy and the health, age, race and education of the mother
Looking at the last sentence of this quote, is this statement addressing potential moderators? potential mediators? or potential third variable problems?