protected double computeConfidence() {
    // num_elt_values() would be more appropriate
    // int num_values = ((PptSlice2) ppt).num_elt_values();
    int num_values = ppt.num_samples();
    if (num_values == 0) {
      return Invariant.CONFIDENCE_UNJUSTIFIED;
    } else {

      return 1 - Math.pow(.5, num_values);
    }
  }
  protected double computeConfidence() {

    return 1 - Math.pow(.5, ppt.num_samples());
  }