In manufacturing, one measure of the efficiency and quality of a process is its sigma rating – which can refer either to yield or the percentage of defect-free products it creates. The number in front of the sigma symbol (σ) indicates where the process’s output lies on the bell curve of standard deviation. 1σ is terrible, with 69% of products coming off the production line with defects – a massive waste of resources. 2σ would see around a 31% defect rate. 3σ would be about 7%.