You can divide estimators of process standard deviation by an unbiasing constant to make them unbiased estimators. They are determined by sample size, and are valid for both small and large samples. Both capability analysis and control charts can use biased or unbiased estimators of process standard deviation in their calculations. Often, the choice to use them depends on company policy or industry standards. By default, Minitab includes unbiasing constants in its calculations.
Unbiasing constants include c4, c5, d2, d3, d4, and c4-prime. See Methods and Formulas in Minitab for a list of their values for various sample sizes.
Unbiasing constants are also called bias correction factors.