Lots of news outlets picked up a  study in JAMA Pediatrics whose results demonstrated a correlation between length of breastfeeding and IQ in children.

Here's the way the story was covered by NBCNews. NBC used the headline, "Kids who were breastfed have higher IQ's, study shows."

a) The headline used by NBC contains two variables. What are they?

b) Is this headline making a causal claim or an association claim?

If you read the text of the NBC story, you get a fairly detailed picture of the variables in this study. For example, the variable "Breastfeeding" was apparently analyzed in two ways–as a categorical vairable (breastfeeding vs. not) and as a quantitative variable (how long babies were breastfed):

Young children who were breastfed as infants scored higher on
intelligence tests than formula-fed kids, and the longer and more
exclusively they were breastfed, the greater the difference, say Harvard
University researchers in a study published today in JAMA Pediatrics.

According to the NBC story,

The Harvard study, unlike most past studies, controlled for these and
other variables, including the mother’s intelligence, education level,
and any postpartum depression; family income and home environment; and
the child’s race, ethnicity, sex and birth weight.

“As a result,
we felt we were able to get a reasonable estimate of what the
relationship is between the length of breastfeeding and the IQ of the
child at school age,” says Dr. Mandy Belfort, lead author and assistant
professor of pediatrics at Harvard Medical School.

The language above should provide a big clue that the researchers used multiple regression in their analyses, because it says, "controlled for."

c) Why would the researchers want to control for mother's intelligence when they look for the association between breastfeeding and child's IQ?

d) Using the information in the two paragraphs quoted above, sketch a regression table. Place the dependent variable at the top, and list this study's independent variables underneath it. Which beta is the focus of the story? What do you know about this beta–was it significant, or not?  What do you think the other betas on the table might be?

 

One of the findings of the study was that moms who breastfed for an entire 12 months had children whos verbal IQ was 4.2 points higher than non breastfed babies. A doctor was quoted by NBC as follows:

“I would take three or four IQ points any given day,” says pediatrician
Michael Georgieff, director of the Center for Neurobehavioral
Development at the University of Minnesota Medical School. Georgieff
was not involved in the current study. “It’s a pretty significant
shift, especially demographically across the world if everyone were to
make that gain.”

This pediatrician is discussing the effect size of this finding. He is suggesting that the effect size, in practical terms, is very important.

e) In your own words, what does the textbook say about the relationship between effect size and the importance of a finding?