  Much work has been devoted to the study of nonstrange baryons belonging to
the $[\textbf{70},1^-]$ multiplet in the framework of the $1/N_c$ expansion.
Using group theoretical arguments here we examine the relation between the
exact wave function and the approximate one, customarily used in applications
where the system is separated into a ground state core and an excited quark. We
show that the exact and approximate wave functions globally give similar
results for all of mass operators presented in this work. However we find that
the inclusion of operators acting separately on the core and on the excited
quark deteriorates the fit and leads to unsatisfactory values for the
coefficients which encode the quark dynamics. Much better results are obtained
when we include operators acting on the whole system, both for the exact and
the approximate wave function.
