  In the next decade several experiments will attempt to determine the neutrino
mass hierarchy, i.e. the sign of $\Delta m_{31}^2$. In the last years it was
noticed that the two hierarchies are disjoint hypotheses and, for this reason,
Wilks' theorem cannot be applied: this means that
$\Delta\chi^2=\chi^2_{IH}-\chi^2_{NH}$ does not follow a one-degree-of-freedom
chi-square distribution. It was proven that, under certain assumptions, it
follows instead a Gaussian distribution with $\sigma=2\sqrt{\mu}$. I will
present several possible definition of sensitivity and review the approaches
proposed in the literature, both within the Bayesian and the frequentist
framework, examining advantages and disadvantages and discussing how they
should be modified if the conditions for Gaussianity are not fulfilled. I will
also discuss the possibility of introducing a new pull parameter in order to
avoid the issue related to the non-nested hypotheses and the differences
between marginalization and minimization, showing under which conditions the
two procedures yield the same $\Delta\chi^2$.
