This shows the basic composition of a control chart. The red dashed lines indicate the range of expected variability 99.73% of the time. If a single point goes outside of these limits, you are probably safe in reacting as if your process has changed in an unusual way. There are other ways to determine if a process change has occurred, called “pattern rules.” Here are the pattern rules LWI uses:
Runs — eight or more points in a row on one side of the centerline
Trends — six or more points in a row decreasing or increasing
Cyclical Patterns — a repeating pattern
Alternating Values — 14 or more points in a row that alternate direction, e.g., up/down/up etc.
Excessive Variability — eight or more consecutive points outside the center third
Lack of Variability — 14 or more points in the center third (near the centerline)