When would you use D-optimal designs? When you have a limited budget and cannot run a completely replicated factorial design. For example, suppose you want to study the response to three factors: A with three levels, B with four levels, and C with eight levels. One complete replication of this experiment would require 3 x 4 x 8 = 96 points (we use the word ‘point’ to mean an experimental unit). Suppose you can afford only 20 points. Which 20 of the 96 possible should you use? The D-optimal design algorithm provides a reasonable choice.