C1.2 (i) There are 1,388 observations in the sample. Tabulating the variable cigs shows that 212 women have cigs > 0.
(ii) The average of cigs is about 2.09, but this includes the 1,176 women who did not smoke. Reporting just the average masks the fact that almost 85 percent of the women did not smoke. It makes more sense to say that the “typical” woman does not smoke during pregnancy; indeed, the median number of cigarettes smoked is zero.
(iii) The average of cigs over the women with cigs > 0 is about 13.7. Of course this is much higher than the average over the entire sample because we are excluding 1,176 zeros.
(iv) The average of fatheduc is about 13.2. There are 196 observations with a missing value for fatheduc, and those observations are necessarily excluded in computing the average.
(v) The average and standard deviation of faminc are about 29.027 and 18.739, respectively, but faminc is measured in thousands of dollars. So, in dollars, the average and standard deviation are $29,027 and $18,739.
C1.3 (i) The largest is 100, the smallest is 0.
(ii) 38 out of 1,823, or about 2.1 percent of the sample. (iii) 17 (iv) The average of math4 is about 71.9 and the average of read4 is about 60.1. So, at least in 2001, the reading test was harder to pass. (v) The sample correlation between math4 and read4 is about .843, which is a very high degree of (linear) association. Not surprisingly, schools that have high pass rates on one test have a strong tendency to have high pass rates on the other test. (vi) The average of exppp is about $5,194.87. The standard deviation is $1,091.89, which shows rather wide variation in spending per pupil. [The minimum is $1,206.88 and the maximum is $11,957.64.] 2.4 (i) When cigs = 0, predicted birth weight is 119.77 ounces. When cigs = 20, = 109.49. This is about an 8.6% drop.