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.