Confidence Limits:
The confidence or control limits are calculated with the help of a statistical measure known as standard deviation (σ).
x̅ is the mean value of the x values for the sample pieces, (x- x̅) is the deviation of an individual value of x, and n is the number of observations.
After calculating the value of standard deviation, the upper and lower control limits can be decided.
- ‘1σ’ limits occupy 68.27% of the area of the normal curve and indicate that one is 68.27% confident that a random observation will fall in this area.
- ‘2σ’ and ‘3σ’ limits occupy 95.45% and 99.73% area of the normal curve and possess a confidence level of 95.45% and 99.73%. In statistical quality control.
- In general practice, ‘3σ’ limits are selected and they are termed as control limits. They present a band (MN) within which the average dimensions of (sample) components are expected to fall. ‘2σ’ limits are sometimes called warning control limits.
