incomplete block design
an experimental design in which treatments are grouped into sets or “blocks,” not all of which include every treatment, and each block is administered to a different group of participants. Incomplete block designs often are employed to avoid administering too many treatment conditions to the same group of participants, which may contribute to undesirable error. For instance, the following arrangement of four treatments (A, B, C, D) is an incomplete block design: Block 1 = A-B-C; Block 2 = A-B-D; Block 3 = A-C-D; and Block 4 = B-C-D. In this particular design, each block contains three treatments, each treatment occurs three times across all blocks, and each pair of treatments occurs together two times across all blocks. Compare complete block design. See block design.