code: public double errorMembership( double[] sampleA ) {
          double[] eig = sampleToEigenSpace(sampleA);
          double[] reproj = eigenToSampleSpace(eig);
  
  
          double total = 0;
          for( int i = 0; i < reproj.length; i++ ) {
              double d = sampleA[i] - reproj[i];
              total += d*d;
          }
  
          return Math.sqrt(total);
      }
docs: <p>
  The membership error for a sample.  If the error is less than a threshold then
  it can be considered a member.  The threshold's value depends on the data set.
  </p>
  <p>
  The error is computed by projecting the sample into eigenspace then projecting
  it back into sample space and
  </p>
  
  @param sampleA The sample whose membership status is being considered.
  @return Its membership error.