Since uniformity is of primary interest in powder blend validation and because of a concern that a constant sampling error can occur, one approach is to base the criteria only on variability. The SDPI allows one to predict, from a sample of size n and with a specified level of assurance, an upper bound on the standard deviation of a future sample of size m from the same population. This approach is recommended in the PDA paper on blend uniformity.
By setting the future sample size m to 10, which is the stage-1 sample size for the USP 25 content uniformity test, and by requiring that the upper bound on the standard deviation of a future sample of size 10 be less than 6.0%, which is the USP 25 stage-1 RSD requirement, the SDPI approach can be tied to the USP 25 content uniformity test. The SDPI equation in Sec. II can be rearranged to obtain the following equation:-
scr = sm/[F1−α,m−1,n−1]1/2
n = size of current sample
scr = critical standard deviation
sm = upper bound of a future sample of size m
1 − α = confidence level (e.g., 0.90)
F = critical F value
Scr becomes the maximum acceptable sample standard deviation to meet the acceptance criteria. If the sample standard deviation Sn is less than Scr, then we are guaranteed, with a minimum assurance of 100(1 − α)%, that the upper prediction bound for a future sample of size 10 will not be greater than 6.0% of the target concentration.