A voice robot beats call center with 8% vs. 6% promo code activations. But how many calls do you actually need to prove the difference is real? We walk through the A/B test math and find the minimum sample size for 95% confidence using Goal Seek in Google Spreadsheets, Z-score and SD.

A contractor claims every fifth call has a bad connection. How many good calls in a row do you need to disprove him? This post uses a binomial hypothesis test to calculate the exact sample size for 99% confidence — with the Google Sheets formula included.

The phone in the sales department has been quiet for half an hour. Broken line, or just a long lull? Every call carries a hidden parameter – the pause before it. Count the 52 longest pauses in a year and you get a threshold: 37 minutes.