Find the average of *fatheduc* in the sample. Why are only 1,192 observations used to compute this average?

a) Mean = ( frac∑_i=1^1192 textfatheduc_i/1192 ), missing data

b) Mean = ( frac∑_i=1^1192 textfatheduc_i/1192 ), outliers removed

c) Mean = ( frac∑_i=1^1192 textfatheduc_i/1192 ), normal distribution assumption

d) Mean = ( frac∑_i=1^1192 textfatheduc_i/1192 ), sampling error adjustment