Often industrial experiments require good fractional factorial designs to examine the effects of many factors by using only a small number of experimental runs. These experimental runs can be determined by assigning factors to the columns of appropriate orthogonal arrays. When the experimental runs are carried out in a time order sequence, the responses can depend on the run order. Frequently level changes are more expensive for some factors in the study than for others. To avoid unwanted time effects and to reduce costs, information is needed about the columns of the orthogonal arrays to assign factors to appropriate columns. In this paper, we review some useful properties of columns in the arrays and present several rules for the assignments. These are helpful in designing experiments using orthogonal arrays. For illustration, several examples are given after these rules have been presented.
