def score_ss
      if @master
        # Find the voxel-indices for the master where the decisions are 1 and 0:
        pos_indices, neg_indices = (@master.narray.eq 1).where2
        @volumes.each do |bin_vol|
          narr = bin_vol.narray
          bin_vol.sensitivity = (narr[pos_indices].eq 1).where.length / pos_indices.length.to_f
          bin_vol.specificity = (narr[neg_indices].eq 0).where.length / neg_indices.length.to_f
        end
      end
    end
def score_ss
      if @master
        # Find the voxel-indices for the master where the decisions are 1 and 0:
        pos_indices, neg_indices = (@master.narray.eq 1).where2
        @volumes.each do |bin_vol|
          narr = bin_vol.narray
          bin_vol.sensitivity = (narr[pos_indices].eq 1).where.length / pos_indices.length.to_f
          bin_vol.specificity = (narr[neg_indices].eq 0).where.length / neg_indices.length.to_f
        end
      end
    end
Scores the volumes of the BinMatcher instance against the reference
 (master) volume, by using Sensitivity and Specificity. The scores are
 stored in the 'sensitivity' and 'specificity' attribute of each binary
 volume instance.

 @see http://en.wikipedia.org/wiki/Sensitivity_and_specificity