The Sheth–Tormen approximation extends the Press–Schechter formalism by assuming that halos are not necessarily spherical, but merely elliptical. The distribution of the density fluctuation is as follows: f ( σ σ r ) = A 2 a π π [ 1 + ( σ σ r 2 a δ δ c 2 ) 0.3 ] δ δ c σ σ r exp ⁡ ⁡ ( − − a δ δ c 2 2 σ σ r 2 ) , where δ δ c = 1.686 , a = 0.707 , and A = 0.3222 . The parameters were empirically obtained from the five-year release of WMAP.