docs: public static double StdDev( int[] values, double mean ){
          double  stddev = 0;
          double  diff;
          int     hits;
          int     total = 0;
  
          // for all values
          for ( int i = 0, n = values.length; i < n; i++ )
          {
              hits = values[i];
              diff = (double) i - mean;
              // accumulate std.dev.
              stddev += diff * diff * hits;
              // accumalate total
              total += hits;
          }
  
          return ( total == 0 ) ? 0 : Math.sqrt( stddev / (total - 1) );
      }