public double getDeviation() {
    double variance = (currentSqrSum - (currentSum * currentSum) / (double) (numberOfDataValues))
      / numberOfDataValues;
    return Math.sqrt(variance);
  }
Returns deviation of the data values that are in the current list of data values
hbase-server/src/main/java/org/apache/hadoop/hbase/util/RollingStatCalculator.java