This comparative fit index computes the relative reduction in the discrepancy function per degree of freedom when moving from the "Null Model" to the k'th model. It is computed as:

(129)

= degrees of freedom for the "Null Model"

= degrees of freedom for the k'th model

= the discrepancy function for the "Null Model"

= the discrepancy function for the k'th model

Comparing Equations 129 and 126, we see that,
for even moderate *N, *there is bound to be virtually no difference
between Bollen's *Rho *and the Bentler-Bonnet Non-normed fit index.