Note, that in the limit of large perturbations σ σ ( M ) ≫ ≫ δ δ M , we expect all matter to be contained in halos such that f ( δ δ M > δ δ c ) = 1 . However, the above equation gives us the limit f ( δ δ M > δ δ c ) = 1 2 . One can make an ad-hoc argument and say that negative perturbations are not contributing in this scheme so that we are mistakenly leaving out half of the mass. And so, the Press-Schechter ansatz is F ( > M ) = erfc ⁡ ⁡ ( 1 2 δ δ M σ σ ( M ) ) ,