The goodness factor is a metric developed by Eric Laithwaite to determine the 'goodness' of an electric motor.
[1][2] Using it he was able to develop efficient magnetic levitation induction motors.
[3] where From this he showed that the most efficient motors are likely to be relatively large.
However, the equation only directly relates to non-permanent magnet motors.
Laithwaite showed that for a simple induction motor this gave: where p is the pole pitch arc length, ρr is the surface resistivity of the rotor and g is the air gap.