checkArgument(values.hasNext(), "Cannot take mean of 0 values");
    long count = 1;
    double mean = checkFinite(values.next().doubleValue());
    while (values.hasNext()) {
      double value = checkFinite(values.next().doubleValue());
      count++;
      // Art of Computer Programming vol. 2, Knuth, 4.2.2, (15)
      mean += (value - mean) / count;
    }
    return mean;
  }