public static double deviation(Iterator tuples,String field){
  return deviation(tuples,field,DataLib.mean(tuples,field));
}

public static double deviation ( Iterator tuples , String field ) { return deviation ( tuples , field , DataLib . mean ( tuples , field ) ) ; }
get the standard deviation of a tuple data value . if any tuple does not have the named field or the field is not a numeric data type , nan will be returned .
