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Both effective diameter and jet-to-plate criteria are required but the approach is easy. 12 April 2018 Inhalation A practical method for eliminating type I and type II errors when assessing the suitability for continued use of a cascade impactor Both effective diameter and jet-to-plate criteria are required but the approach is easy Daryl L. Roberts, PhD a Christopher M. Shelton, MS b a Applied Particle Principles, LLC b West-Ward Pharmaceuticals clusion that the stage was not suitable for continued use in the laboratory. Roberts and Romay 1 first posed the fundamentally based "effective diameter" as a proper averaging of the individual nozzle diameters and linked this metric with the aerodynamic performance of the instrument. is averaging method eliminated the error of disqualifying an acceptable impactor (a Type I error). Although the averaging approach is a scientifically supported basis for evaluating mensuration data, the method may not detect an unacceptable stage in some instances. Roberts and Mitchell 2 recently showed that it is possible for the effective diameter to be within the required range even when there are nozzles that are too small to cause impaction on the collection surface. e so-called "jet-to-plate" criterion posed by these authors identifies such nozzles and other unusual nozzle possibilites, and when applied to the question of impactor suitability, will eliminate the error of qualifying an unacceptable impactor (a Type II error). In this article, we summarize the theory and practice of applying both the "effective diameter test" and the "jet- to-plate test" to the question of the suitability for con- tinued use of a cascade impactor. e overall goal is to help users eliminate both Type I and Type II errors when assessing impactor suitability. Perhaps surpris- ingly, no data other than that already gathered by opti- cal stage mensuration is required to apply both tests, allowing the impactor user community, with no addi- tional measurements, to ensure that impactors in use are aerodynamically equivalent to a new impactor. Introduction Cascade impactors are among the key pieces of analyti- cal equipment in routine use for assessing the quality of inhalable drug products. e difficulty of calibrating impactors with particles of a known size makes the cas- cade impactor rare among analytical instruments. In a Good Manufacturing Practice (GMP) environment, the aerodynamic performance of the impactor must be known before the impactor is released for use in the laboratory. e calibration of new instruments is ide- ally obtained from a published calibration with parti- cles. However, this calibration is laborious and avail- able only at laboratories with specialized equipment, making it impractical for evaluating used impactors. Consequently, once the impactor has been in service, the user needs to make a decision about its continued use without being able to measure experimentally the current state of the particle size fractionation ability of the impactor. This situation has led to a surrogate measurement, namely optical quantification of stage nozzle diameters (stage mensuration), as the preferred method of assess- ing instrument calibration. Early practitioners of this method, however, found themselves faced with the common problem of nozzles on a stage that were smaller (or larger) than the manufacturer's individual nozzle specification for a new impactor. Although it may have been logical to assume that the presence of a single nozzle diameter out of specification on a stage with typically hundreds of nozzles would not change the stage performance, the absence of a scientific ratio- nale for accepting such a stage often resulted in the con-