Experimental Designs Optimally Balanced for Trend

Abstract

When experiments are carried out over a period of time, the response may be subject to time trends. We use an algorithm for exact optimum designs to construct a series of designs resistant to linear and quadratic trend. Designs considered include the allocation of simple treatments, multifactor designs in qualitative or quantitative factors, and response surface designs. The investigation is extended from consideration of linear ordering in time to include designs with several trials at each time point, designs in which several trials are spread over only one out of three shifts per day, and designs in which there are more time points than experiments. Comparisons with designs in the absence of trend show that surprisingly little information is lost by designing for protection against a potential trend. The BT algorithm for obtaining these designs is outlined.

KEY WORDS:

• Adjustment algorithm
• BLKL exchange algorithm
• BT algorithm
• Design efficiency
• Optimum design
• Response surface
• Trend-free design