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Tutorials : Feb 15, 2012 (Vol. 32, No. 4) ## Practical Interpretation of Equivalent Acceptance Criteria##
A Useful Strategy to Assist in Comparability Criteria Determination
A quality practitioner may be interested in assessing whether two process means are statistically equivalent, e.g., whether two processes (an historical process and a new process) produce equivalent results for a quality attribute. Statistical equivalency tests, e.g., two one-sided t-tests (TOST) are widely accepted as an acceptable method for demonstrating equivalency. In contrast with traditional hypothesis testing approaches, where the null hypothesis assumes equality across two parameters of interest (e.g., equal means), the null hypothesis using TOST can assess whether the average difference exceeds a comparability criteria known as the EAC (equivalency acceptance criteria) and can be written as: H H where µ To show average equivalency a 90% two-sided confidence interval for the difference of two means must fall completely within the range from –EAC to EAC. In some instances this region may be mandated, e.g., using 80% to 125% as is required in bioequivalence testing. In most cases, the EAC is developed with subject matter experts. This article describes a graphical approach based on the work of Burdick et al., (published in Assume we have n A sample of size n Rewriting equation (3) from Burdick et al., we will be able to conclude average equivalency using the TOST procedure if the sample mean calculated with a sample of size n where ME is the margin of error. Assuming equal but unknown variances, the ME can be calculated as: where t is the upper α/2 value from a t distribution with n For example, assume there are n UEDL = 44.7 + 8 – (1.686) x (3.4) x √(1/10) = 50.9 LEDL = 44.7 – 8 + (1.686) x (3.4) x √(1/10) = 38.5 These results can be shown alongside the historical values as illustrated in Using the UEDL and LEDL values, the utility of the chosen EAC can be objectively assessed before data from the new process are collected. This methodology may be of particular use in the planning stage for evaluating EAC values when there are no pre-determined criteria available. Keith M. Bower ([email protected]) is a principal quality engineer in global quality engineering at Amgen. Web: www.amgen.com. |

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