Split-Plot Designs

Experimental Objective

This example revisits the tablet production scenario in Chapter 3, "Full Factorial Designs," with an alternate analysis suggested by Fossceco (1999). The tablet production process has several stages, which include batch mixing, where ingredients are combined and mixed with water, and pellet production, where the batch is processed into pellets that are compressed to form tablets. It is more convenient to mix batches and randomize the treatments within each batch than to mix a new batch for each run. Thus, this experiment calls for a standard two-stage split-plot design.

The moisture content and mixing speed for the batch constitute whole-plot factors, while the factors that control the variety of ways that pellets can be produced from a single batch are subplot factors. The responses of interest are measured on the final tablets. Table 12.1 shows all the variables involved and the stage with which they are associated.

Table 12.1: Factors and Responses in the Tablet Formulation Experiment

Variable	Name	Low	High	Description
		Level	Level
Whole Plot	FLUID	90	115	Moisture content (%)
	MIX	15	45	Mixing time (min)
Split Plot	EXTRUDER	36	132	Extruder speed (rpm)
	SCREEN	0.6	1.0	Screen size (mm)
	RESID	2	5	Residence time (min)
	DISK	450	900	Disk speed (rpm)
Response	MPS			Mean particle size (micron)

The goal of the experiment is to determine which effects are significant. The researcher is interested in both whole-plot factors and subplot factors.

---

Task List