In<unk> s, an<unk>  is a<unk>  whose probability distribution does not depend on which of the probability distributions among those being considered is the distribution of the<unk> al population from which the data were taken.  This concept was introduced by the great<unk> al geneticist Sir Ronald Fisher.
Ancillary statistic
Absolute deviation
Hazard ratio
Degrees of freedom 
Quantum yield
Allan variance
Non-parametric statistics
Attributable risk