public class Statistics {

  long numSamples = 0;
  double sampleTotal = 0;
  double sampleMean = 0;
  double sampleVar = 0;
  
  public void addData (double x)
  {
    // Increment number of samples obtained.
    numSamples ++;

    // Accumulate total for sample mean.
    sampleTotal += x;
    // First, save current mean.
    double oldMean = sampleMean;
    // Now update sample mean.
    sampleMean = sampleTotal / numSamples;

    // Use recurrence relation for variance.
    if (numSamples == 1) {
            sampleVar = 0;
    }
    else {
      sampleVar = (1 - 1.0 / (double) (numSamples - 1)) * sampleVar 
      + numSamples * (sampleMean - oldMean) * (sampleMean - oldMean);
    }
  }

  public double getMean ()
  {
    return sampleMean;
  }

  public double getVariance ()
  {
    return sampleVar;
  }

  public double getConfidenceHalfInterval ()
  {
    // mean +- this value, i.e., this is half the interval.
    return 1.96 * Math.sqrt (sampleVar / numSamples);
  }

  public String toString()
  {
    if (numSamples == 0) 
      return "No samples";
    else 
      return getMean() + " +- " + getConfidenceHalfInterval() + "  #samples=" + numSamples;
  }

}