The file NCbirths contains data on a random sample of 1450 birth records in the state of North Carolina in the year 2001. This sample was selected by John Holcomb, based on data from the North Carolina State Center for Health and Environmental Statistics. One question of interest is whether the distribution of birth weights differs among mothers’ racial groups. For the purposes of this analysis, we will consider four racial groups: white, black, Hispanic, and other (including Asian, Hawaiian, and Native American). Use the variable MomRace, which gives the races with descriptive categories. (The variable RaceMom uses only numbers to describe the races.) The response was the birth weight of the a newborn infant, and the factor of interest was the racial/ethnic group of the mother.
(a) Explain why random assignment is impossible, and how this impossibility limits the scope of inference. Why is inference about cause not possible?
(b) Under what circumstances would inference from samples to populations be justified? Were the requirements for this kind of inference satisfied?