Nearly Orthogonal Arrays with Mixed Levels and Small Runs

In running a factorial experiment, it may be desirable to use an orthogonal array with different (mixed) numbers of factor levels. Because of the orthogonality requirement, such arrays may have a large run size. By slightly sacrificing the orthogonality requirement, we can obtain nearly orthogonal arrays with economic run size. Some general methods for constructing such arrays are given. For 12, 18, 20, and 24 runs, many orthogonal arrays and nearly orthogonal arrays with mixed levels are constructed and tabulated. Effects of near orthogonality on estimation efficiency and analysis are studied.