I am attempting to predict college enrollment using variables that are "piped" responses, where answers from a previous question are piped into subsequent questions. For example, I have a variable on whether students took the SAT (yes/no) and a piped response on whether those taking the SAT scored above 1250 or not. I was initially trying to treat taking and passing the SAT as polychotomous, but either that's not possible, or I don't know how to do it. The problem results in the subsequent question, where I'm left with only information on those who took the test (and not on those who did not). The piped variable has a "0" for a score below 1250, a "1" for above 1250 and blanks for those who did not take the test. If I try to use both variables as predictors, the piped variable reduces the n size in the analysis to about 1/3 because very few students actually took the SAT. Is there a way to model this "piped" data using dummy variables so that I don't reduce the n size so dramatically?
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