code: function(zCriticalValue) {
          var baselineValue = this.baseline.pEstimate(zCriticalValue).value;
          var ratio = this.differenceEstimate(zCriticalValue).value / baselineValue;
          var error = this.differenceEstimate(zCriticalValue).error / baselineValue;
          return new Abba.ValueWithError(ratio, error);
      }
docs: Return the difference in sucess rates as a proportion of the baseline success rate.