σ is a Greek letter, sigma in English and Sigma in Chinese. The term σ is used to describe the distribution or dispersion of the average value of any process parameter.

The meaning of σ applies to business or manufacturing process.

For business or manufacturing process, σ value is a measure to indicate the working condition of the process. The higher the σ value, the better the process conditions. σ value is used to measure the ability of the process to complete defect-free operation, because defects will lead to customer dissatisfaction in any case. In other words, the σ value indicates the frequency of defects, and the higher the σ value; The lower the process defect rate. When the σ value increases, the defective rate decreases, the quality cost decreases, the process cycle time shortens, and the customer satisfaction improves. When the σ value reaches 6, that is, the quality of 6σ means "only 3.4 defective units per million", and the long-term quality compliance rate is 99.99966%. Relatively speaking, when the σ value is only 3, that is, the quality of 3σ means "there are 66,807 defective products per million units", and the qualified rate is 93.32%. Chemically, surface tension (σ)(sigma) refers to the force that molecules on the surface of a liquid shrink inward to the smallest area. Chemically, it also refers to the σ bond formed by two electron clouds "head to head" along the orbital axis. For average sum and stability, there must be a specific mathematical value to quantify its stability or not. σ is a specific mathematical value used to quantify the degree of stability and instability, which is itself a concept of error analysis in statistics. It is borrowed here to indicate the quality. When applied to enterprise management, σ represents the standard deviation, and the smaller the σ value, the more stable the label quality level; On the contrary, the greater the σ value, the more unstable the quality level is. If a company needs a stable process, it should try its best to reduce the σ value. Statistically speaking, σ value is a measure of process, and the specific calculation of σ value will be explained in the course of green belt and black belt in the future. Every result is produced by a process. The process is repeated, but there will be some subtle differences in the output of the process, which is called fluctuation. Only by controlling the difference of these outputs and making them change within a reasonable range can the quality of products be guaranteed. Fluctuation is also called dispersion degree. Due to the difference of dispersion and concentration, fluctuation is the standard measure of process quality. In the past, the average value was used to measure the quality of the process, but now σ is added to measure the degree of dispersion and concentration. In this way, the process can be evaluated more ideally and comprehensively. In HPLC, σ refers to the standard deviation, which is half the peak width (inflection point) of chromatographic normal distribution curve x = 1. The inflection point of normal peak width is 0.607 times of peak height. The standard deviation indicates the dispersion of components in the process of flowing out of the chromatographic column. Small σ, small dispersion, high polar concentration, fine peak shape and high column efficiency; On the contrary, σ is large, the peak shape is fat and the column efficiency is low.