code: public static double xbarStd(TransposeDataCollection sampleDataCollection, AssociativeArray nh, AssociativeArray populationNh) {
          return Math.sqrt(xbarVariance(sampleDataCollection, nh, populationNh));
      }
docs: Calculates Standard Deviation for Xbar
  
  @param sampleDataCollection
  @param nh
  @param populationNh
  @return