Man (in the generic sense), Machine, Material, Method, Measurement, Mother Nature are the categories/the components of variation that contribute to the production of some desired result.
Use: There may be thousands of factors that directly affect or interact with each other to affect the outcome of some desired result(s) of a process. The 6M’s started with Dr. Ishikawa as the 4M’s (Machine, Material, Method, Measurement) and over time it evolved to including the human element and noise factors affected by the forces of nature (e.g., humidity, vibration, temperature, etc.) The team uses these 6M’s as memory joggers to focus on when they are identifying as many of the factors that might influence the process as possible. Typically, they will use a Fishbone Diagram along with brainstorming to gather these ideas. After the team has exhausted the brainstorming session, the team will then boil down the many factors to the vital few factors from which they will place their monitoring, controlling, and improvement methodologies.
