In search of the √N + 1 Rule

In over 25 years of consulting in operations research, management science, quality management, and statistics, I have come across several rules of thumb that practitioners use frequently, but incorrectly. One is the ‘square root of (N) + 1’ rule used in sampling and inspecting raw materials and finished goods. This paper describes several of my encounters with this rule and the circumstances of its use. I discuss how I used a nested design experiment to demonstrate to users that the rule was not appropriate for their situations. The paper also includes a ‘top 10’ list of other rules of thumb and beliefs that people often use incorrectly. I offer tips that have helped me avoid these rules.