docs: private long[] determinePreferenceVector(Relation<V> relation, DBIDRef id, DBIDs neighborIDs) {
      NumberVector p = relation.get(id);
      // variances
      final int size = neighborIDs.size(), dim = p.getDimensionality();
      double[] sumsq = new double[dim];
      for(DBIDIter iter = neighborIDs.iter(); iter.valid(); iter.advance()) {
        NumberVector o = relation.get(iter);
        for(int d = 0; d < dim; d++) {
          final double diff = o.doubleValue(d) - p.doubleValue(d);
          sumsq[d] += diff * diff;
        }
      }
  
      // preference vector
      long[] preferenceVector = BitsUtil.zero(dim);
      for(int d = 0; d < dim; d++) {
        if(sumsq[d] < alpha * size) {
          BitsUtil.setI(preferenceVector, d);
        }
      }
      return preferenceVector;
    }