SCOPE: utility to compute percent positive lymph nodes

This program removes the cases with zero lymph nodes examined and compute the positive LN density.

This program asks the user to select these two variables as well as the observedoutcome variable. It is assumed that these three are the only variables in the working file.
NOTE: Due to the very large data set of SEER data over many disease sites,
specialized SCOPE utility like this one can be very useful and flexible for the user.
This program find and remove the cases with the zeros in the total number of lumph node column, so that a new variable called lymph node density = positive LN/total number of LN can be computed.