<math>\mathrm{PS}=\frac{P(e\mid c)-P(e\mid C\backslash c)}{1-P(e\mid C\backslash c)}=\frac{suff(e,c)+nec(e,c)-1}{nec(e,c)}</math> | <math>\mathrm{PS}=\frac{P(e\mid c)-P(e\mid C\backslash c)}{1-P(e\mid C\backslash c)}=\frac{suff(e,c)+nec(e,c)-1}{nec(e,c)}</math> |