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A trend in s o w a r e development is to assemble a system from a number of components. These may be either available commercially off-the-sheK or by the use of network-based resources. In many cases, the system is expected to function f o r multiple configurations of interchangeable components. The trade off that a system tester faces is the thoroughness of test component configuration coverage, versus limited resources of time and expense. This paper presents a metric that can be used to measure component interaction coverage of a set of system test configurations. We also provide a formal dejnition of the system test interaction problem.

Covering arrays with mixed alphabet sizes, or simply mixed covering arrays, are natural generalizations of covering arrays that are motivated by applications in software and network testing. A (mixed) covering array A of type Q k i¼1 g i is a k  N array with the cells of row i filled with elements from Z gi and having the property that for every two rows i and j and every ordered pair of elements ðe; f Þ 2 Z gi  Z gj , there exists at least one column c, 1 c N, such that A i;c ¼ e and A j;c ¼ f . The (mixed) covering array number, denoted by cað Q k i¼1 g i Þ, is the minimum N for which a covering array of type Q k i¼1 g i with N columns exists. In this paper, several constructions for mixed covering arrays are presented, and the mixed covering array numbers are determined for nearly all cases with k ¼ 4 and for a number of cases with k ¼ 5.