code: @Override
  	public double calculateAgreement() {
  		double D_O = calculateObservedDisagreement();
  		double D_E = calculateExpectedDisagreement();
  		if (D_O == D_E)
  			return 0.0;
  		else
  			return 1.0 - (D_O / D_E);
  	}
docs: /* Calculates the inter-rater agreement for the annotation
  study that was passed to the class constructor and the currently
  assigned distance function.
  @throws NullPointerException if the study is null.
  @throws ArithmeticException if the study does not contain any item or
  the number of raters is smaller than 2.