A baby weighing more than 4,000 g at birth is considered to be large for gestational age (LGA). Gestational diabetes in the mother is believed to be a common risk factor for LGA. In an article published in the New England Journal of Medicine (October 2009), researchers Landon et al. reported a study of 958 women diagnosed with mild gestational diabetes between 24 and 31 weeks of pregnancy who volunteered to participate in the study and then were randomly assigned to one of two groups: 473 to usual prenatal care (control group) and 485 to dietary intervention, self- monitoring of blood glucose, and insulin therapy, if necessary (treatment group). Of the 473 women in the control group, 68 had babies who were LGA, and of the 485 women in the treatment group, 29had babies who were LGA.
a. Was this an experiment or an observational study? Explain how you are deciding.
b. Identify the observational units.

c. Identify the explanatory and the response variables. Also, for each variable, be sure to identify the kind of variable (categorical or quantitative).
d. Organize the data in a well-labeled 2 × 2 table. Also create a segmented bar chart with well-labeled axes to display the data. Discuss what the segmented bar chart shows with regard to a relationship between the type of prenatal care a woman with gestational diabetes receives and whether or not she has an LGA baby.
e. Find the observed difference in the conditional proportion of mothers who had LGA babies between those who received the usual prenatal care and those who received specialized care.
f. Give two possible explanations for the observed difference in conditional proportions as reported in part (e).