Quality Control Sample Size Calculator

The standard statistical sample size formula is: n = Z^2 x p x (1-p) / e^2, where Z is the z-score for the desired confidence level (1.96 for 95%), p is the expected proportion defective (use 0.5 if unknown, which gives the most conservative estimate), and e is the acceptable margin of error (e.g., 0.05 for plus or minus 5%).

AQL (Acceptable Quality Level) is the maximum defect rate considered acceptable in a lot, per ANSI/ASQ Z1.4. The AQL table specifies code letters based on lot size and inspection level (I, II, or III), and the code letter maps to a sample size. This is the standard approach for incoming inspection in manufacturing.

Confidence level (e.g., 95%) is a statistical concept: if the same inspection were repeated many times, 95% of the intervals would contain the true proportion defective. AQL is a contractual acceptance criterion that defines the defect rate a customer is willing to accept. They serve different purposes in quality planning.

Apply the finite population correction when the sample is a significant fraction of the lot. Corrected n = n0 / (1 + (n0 - 1) / N), where n0 is the initial sample size and N is the lot size. If n0 / N is less than 5%, the correction is negligible.

Inspection Level II is the default level specified in ANSI/ASQ Z1.4 for normal inspection. Level I uses a smaller sample (lower discrimination), and Level III uses a larger sample (higher discrimination). Special levels S1-S4 are for destructive or costly tests where small samples are needed.