code: function(zCriticalValue) {
          var baselineP = this.baseline.pEstimate(zCriticalValue);
          var variationP = this.variation.pEstimate(zCriticalValue);
          var difference = variationP.value - baselineP.value;
          var standardError = Math.sqrt(Math.pow(baselineP.error, 2) + Math.pow(variationP.error, 2));
          return new Abba.ValueWithError(difference, standardError);
      }
docs: Generate an estimate of the difference in success rates between the variation and the baseline.