ABSTRACT
It is an elementary fact that the size of an orthogonal array of strength t on k factors must be a multiple of a certain number, say Lt, that depends on the orders of the factors. Thus Lt is a lower bound on the size of arrays of strength t on those factors, and is no larger than Lk, the size of the complete factorial design. We investigate the relationship between the numbers Lt, and two questions in particular: For what t is Lt < Lk? And when Lt = Lk, is the complete factorial design the only array of that size and strength t? Arrays are assumed to be mixed-level.
We refer to an array of size less than Lk as a proper fraction. Guided by our main result, we construct a variety of mixed-level proper fractions of strength k − 1 that also satisfy a certain group-theoretic condition.
MATHEMATICS SUBJECT CLASSIFICATION:
Acknowledgments
We thank the associate editor and the referee for their helpful comments.
Notes
1 The condition assumed in Beder and Beder (Citation2014) is that the counting function of the multiset D be constant on conjugacy classes. Since we are constructing proper fractions, D is a set and its counting function is its ordinary indicator function, which is constant on conjugacy classes iff D is a union of such classes.