Optimizely Statistical Calculation for Difference Intervals

https://assets.ctfassets.net/zw48pl1isxmc/4RfbbGWUSAcwEqYu6miCgA/f9d616b7d0953bc818ed129de76ac334/practical_guide_to_stats.pdf

http://pages.optimizely.com/rs/optimizely/images/stats_engine_technical_paper.pdf

https://help.optimizely.com/Analyze_Results/Difference_intervals_and_improvement_intervals