Where does sigma collide?

The Greek letter sigma writes the standard deviation, a single number for how widely the values in a distribution scatter around their mean. Pearson coined the term standard deviation in 1894 in his Contributions to the mathematical theory of evolution, giving one summary of spread computed as the square root of the average squared distance from the mean, and the symbol sigma became its fixed shorthand. Here sigma is a measure of variability in a set of numbers, not a chemical linkage, a drug target, or a quality program.[1]

Unit

One number in the same units as the data, the root-mean-square distance from the mean

Recovery means

Not applicable

Measured by

Computed from data as the square root of the mean squared deviation from the average, or estimated from a sample

In chemistry a sigma bond is a covalent bond whose shared electron cloud is symmetric about the line joining the two atoms, formed when atomic orbitals overlap head-on along that axis. Mulliken introduced the sigma and pi labels for molecular orbitals in his 1932 Electronic Structures of Polyatomic Molecules and Valence. II, classifying orbitals by their symmetry about the bond axis, and a sigma bond is the strongest single link between two atoms because the overlap is direct. Here sigma names a physical connection between atoms, not a statistical spread, a receptor in the brain, or a manufacturing method.[2]

Unit

One covalent linkage between two atoms, described by bond length and bond energy

Recovery means

Not applicable

Measured by

Inferred from spectroscopy, diffraction geometry, and quantum-chemical calculation of orbital overlap and bond energy

In pharmacology the sigma receptor is a protein binding site in the body that certain drugs latch onto, first proposed by Martin and colleagues in 1976 from experiments in the chronic spinal dog, where a group of drugs typified by SKF-10047 produced effects distinct from other opioid actions and were assigned to a separate sigma site. The site was long confused with others until Hanner and colleagues purified and cloned the sigma-1 protein in 1996, showing it is a genuine molecular target rather than an opioid receptor. Here sigma names a specific protein that drugs bind, not a number for spread, a bond between atoms, or a production standard.[3][4]

Unit

One protein binding site, characterised by its affinity for particular ligands

Recovery means

Not applicable

Measured by

Assayed by radioligand binding, and identified at the molecular level by purification, cloning, and sequencing of the protein

In operations management Six Sigma is a structured method for reducing defects and variation in a process, coined by Smith at Motorola in 1986 and named for the goal of holding a process so tightly that its specification limits sit six standard deviations from the mean, leaving very few defects. Schroeder and colleagues in their 2008 Six Sigma: Definition and underlying theory defined it as an organized, data-driven improvement approach built on trained specialists and structured projects. Here sigma is borrowed from statistics to label a management program, so unlike the pure statistical measure it names a whole way of running and improving work, and it has nothing to do with a chemical bond or a brain receptor.[5]

Unit

One improvement program or project aimed at a defect rate, scaled by process capability

Recovery means

Not applicable

Measured by

Tracked by process-capability indices, defects per million opportunities, and the sigma level of a process