code: public double getCovariance() {
  		if(covariance == null) {
  			Observation o = new Observation();
  			for(int i=0; i<observationA.getObservationCount(); i++)
  				o.addValue(observationA.getValueAt(i)*observationB.getValueAt(i));
  			covariance = o.getExpectation()-observationA.getExpectation()*observationB.getExpectation();
  		}
  		return covariance;
  	}
docs: Return the covariance of the two value distributions.<br>
  The covariance describes the linear relation between two random variables (how much they change together).
  @return The covariance of the two value distributions