This notebook includes code for a PolarizationOperator which allows one to derive the level of polarization for a single spin or group of spins, from an arbitrary density operator in an arbitrary spin system.
The syntax for obtaining the polarization level of spin j from a density operator rho is
OperatorAmplitude[rho -> PolarizationOperator[j] ]
The syntax PolarizationOperator[{j,k..}] allows calculation of the mean polarization over a set of spins.
The syntax PolarizationOperator[] allows calculation of the mean polarization over all spins in the current spin system.
The attached notebook PolarizationOperator.nb contains the necessary definitions, and some examples.