This assignment involves multiple linear regression. The datasets (for question 2) can be found on Sakai: go to Resources \(\rightarrow\) Datasets \(\rightarrow\) Homework Datasets \(\rightarrow\) Methods and Data Analysis 2. Please type your solutions using R Markdown, LaTeX or any other word processor but be sure to knit or convert the final output file to “.pdf”. Submissions should be made on gradescope: go to Assignments \(\rightarrow\) Methods and Data Analysis 2.
Question 1 was taken and adapted from Chapter 7 of Ramsey, F.L. and Schafer, D.W. (2013), “The Statistical Sleuth: A Course in Methods of Data Analysis (3rd ed).”
MATERNAL SMOKING AND BIRTH WEIGHTS. These days, it is widely understood that mothers who smoke during pregnancy risk exposing their babies to many health problems. This was not common knowledge fifty years ago. One of the first studies that addressed the issue of pregnancy and smoking was the Child Health and Development Studies, a comprehensive study of all babies born between 1960 and 1967 at the Kaiser Foundation Hospital in Oakland, CA. The original reference for the study is Yerushalmy (1964, American Journal of Obstetrics and Gynecology, pp. 505-518). The data and a summary of the study are in Nolan and Speed (2000, Stat Labs, Chapter 10) and can be found at the book’s website.
The data for this question can be found in the file “babiesdata.csv” on Sakai.
There were about 15,000 families in the study. We will only analyze a subset of the data, in particular 1236 male single births where the baby lived at least 28 days. The researchers interviewed mothers early in their pregnancy to collect information on socioeconomic and demographic characteristics, including an indicator of whether the mother smoked during pregnancy. The variables in the dataset are described in the code book at the end of this document.
Note that this is an observational study, because mothers decided whether or not to smoke during pregnancy; there was no random assignment to smoke or not to smoke. Thus, we cannot make causal inference statements from the results of a standard regression model.
There are some complexities in the original dataset to be aware of. Some variables have missing values. In particular, you will see from the babiesdata.csv file that the height and weight of the father are missing quite frequently. This is typical in data on births, as it is often difficult to get data about the fathers. I recommend that you not consider father’s height and weight when modeling. Some of the other variables have a few missing cases here and there. For this analysis, you can drop them from the modeling. This is not the ideal way to handle missing data in an analysis–and we will learn better methods later in the course–but for now it will move the analysis forward. I strongly recommend that you make a data file that has complete observations on every single case for all the variables you are thinking about including in the model, and run the regression using that file. For example, I posted such a file in the Sakai site that excludes all of the variables on the fathers. You are welcome to use this file, or make your own with complete cases if you really want to use fathers’ data. The modified data can be found in the file “smoking.csv” on Sakai.
The data files also contain two outcome variables: gestational age and birth weight. Both of these could be affected by smoking, so both are outcomes rather than predictors. It does not make sense scientifically to include one as a predictor of the other; the two variables happen simultaneously and hence are a bivariate outcome. For this analysis, we exclude gestational age from the modeling. Of course, one could do a separate regression for gestational age to see if smoking has an effect on gestational ages. Even better, one could treat birth weight and gestational age as a bivariate outcome and fit a regression model that predicts the bivariate outcome. This is a model we won’t have to time to learn about in our course, but come find the instructor if you want to learn more. The file also contains an indicator variable for Premature (gestational age < 270 days), which is just a recoding of gestational age; we won’t use that.
The main file also includes information on the number of cigarettes smoked and about timing for mothers who quit smoking. For this analysis you do not have to use those variables, as we just compare smokers and non-smokers. Also, for this analysis, you can ignore the birth date variable, you can collapse education categories from 6-7 into one category for education = trade school, and you can also collapse race categories from 0 - 5 into one category for race = white.
Finally, regarding the fathers’ data, you might pay attention to correlation among the mothers’ and fathers’ values. For example, the mothers’ and fathers’ races might tend to be similar (use a “table” command to see the contingency table of the two races), in which case you have to be concerned about effects of multicollinearity if you want to include both mother’s and father’s races in the model.
Code Book
| Variable | Description |
|---|---|
| Id | id number |
| birth | birth date where 1096 = January1, 1961 |
| gestation | length of gestation in days |
| bwt | birth weight in ounces (999 = unknown) |
| parity | total number of previous pregnancies, including fetal deaths and still births. (99=unknown) |
| mrace | mother’s race or ethnicity 0-5=white 6=mexican 7=black 8=asian 9=mix 99=unknown |
| mage | mother’s age in years at termination of pregnancy |
| med | mother’s education 0 = less than 8th grade 1 = 8th to 12th grade. did not graduate high school 2 = high school graduate, no other schooling 3 = high school graduate + trade school 4 = high school graduate + some college 5 = college graduate 6,7 = trade school but unclear if graduated from high school 9 = unknown |
| mht | mother’s height in inches |
| mpregwt | mother’s pre-pregnancy weight in pounds |
| drace | father’s race or ethnicity 0-5 = white 6 = mexican 7 = black 8 = asian 9 = mix |
| dage | father’s age in years at termination of pregnancy |
| ded | father’s education 0 = less than 8th grade 1 = 8th to 12th grade. did not graduate high school 2 = high school graduate, no other schooling 3 = high school graduate + trade school 4 = high school graduate + some college 5 = college graduate 6,7 = trade school but unclear if graduated from high school 9 = unknown |
| dht | father’s height |
| dwt | father’s pre-pregnancy weight in pounds |
| marital | marital status of mother 1 = married 2 = legally separated 3 = divorced 4 = widowed 5 = never married |
| income | family yearly income in 2500 increments. 0 = under 2500, 1 = 2500-4999, …, 9 = 15000+. 98=unknown, 99=not asked |
| smoke | does mother smoke? 0 = never 1 = smokes now 2 = until preg 3 = once did, not now |
| time | If mother quit, how long ago did she quit? 0 = never smoked, 1 = still smokes, 2 = quit during pregnancy, 3 = up to 1 yr ago, 4 = up to 2 yr ago, 5 = up to 3 yr ago, 6 = up to 4 yr ago, 7 = 5 to 9yr ago, 8 = 10+yr ago, 9 = quit and don’t know, 98 = unknown |
| number | number of cigs smoked a day for past and current smokers 0 = never smoked 1 = 1-4 2 = 5-9 3 = 10-14 4 = 15-19 5 = 20-29 6 = 30-39 7 = 40-60 8 = 60+, 9 = smoke but don’t know |
| Premature | 1 = baby born before gestational age of 270, and 0 = otherwise. Ignore this for modeling since it is just a dichotomized function of the gestational age. |
30 points: 10 points for question 1, 20 points for question 2.