created form, want to require input
 

I have created a survey form and grouped the 6 responses
to questions allowing the user to only choose one out of
six as a response. Based on which rating is chosen, I'd
like to require a comment to be provided by user.

More specifically, there are 5 ratings from "Very
satisfied" to "Very unsatisfied" plus one rating
of "N/A". If a user chooses unsatisfied or very
unsatisfied, I'd like to require them to comment (in the
cell below the ratings) why they rated that item poorly.

Any suggestions ? Only thing I could think of was to
check for a TRUE response to either "unsatisfied" or "very
unsatisfied", then check the length of characters in
the "comment" cell to see if the length was 0.

Would this work ? Is there an easier way ?