Initial control limits are established by analyzing controls over how many consecutive days?

• A. 10
• B. 20
• C. 30
• D. 40

Answer: (B)

Establishing initial control limits comes from creating a baseline by collecting measurements to estimate the process average and its dispersion. Using twenty consecutive days provides a stable estimate of the mean and standard deviation, allowing you to set the center line and the upper and lower control limits (often at mean ± 3 SD). This 20-day baseline balances having enough data to smooth out random day-to-day variation with not delaying the detection of real shifts. Once these limits are set, future measurements are evaluated against them to identify out-of-control signals. Using too few days would make the limits unstable and prone to false alarms; using too many would slow down detecting real changes. Hence, twenty consecutive days is the standard baseline for initial limits.