4 The data set BWGHT contains data on births to women in the

4 The data set BWGHT contains data on births to women in the United States. Two variables of interest are the dependent variable, infant birth weight in ounces (bwght), and an explanatory variable, average number of cigarettes the mother smoked per day during pregnancy (cigs). The following simple regres- sion was estimated using data on n = 1,388 births: bwght = 1 19.77-0.514 cigs (i) What is the predicted birth weight when cigs 0? What about when cigs 20 (one pack per (ii) Does this simple regression necessarily capture a causal relationship between the child\'s birth iii) To predict a birth weight of 125 ounces, what would cigs have to be? Comment. day)? Comment on the difference. weight and the mother\'s smoking habits? Explain. The proportion of women in the sample who do not smoke while pregnant is about .85. Does (iv) this help reconcile your finding from part (iii)?

Solution

1) 119.77 is the birth weight when cigs = 0. When cigs =20 > bweight = 119.77 - 10.28 = 109.49

2) No -> This does not capture a causal relationship. A confounding factor would be, excessive smoking makes it difficult for women to conceive. So smoking women might not have a child in the first place.

3) cigs will have to be -10 that is a negative consumption. This is economicall impossible as it is not possible to consume negative quantities of any commodity.

4) Yes. This shows that sample is not random. It has a selection bias in favor of those women whose chances of being expectant improved as a result of being a non-smoker.