Efficient information
= Efficient information
{title2=$\widetilde I=P\widetilde\ell^2$}
The efficient information for a scalar target <statistical parameter> is the squared <L2 norm> of its <efficient score>. It cannot exceed the parametric <Fisher information>, by the <Pythagorean theorem in an inner-product space>. It can vanish when first-order target variation can be reproduced by nuisance variation.