Lesson 14: Factorial Design

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Introduction

A factor is a variable that is controlled and varied during the course of an experiment. In a chemistry experiment, temperature and pressure may be the factors that are deliberately changed over the course of the experiment. In the clinical trial treatment can be a factor. A study of experimental therapy vs. placebo can be thought of as having a treatment factor with 2 levels, 0 or the study dosage. A study with two different treatments has the possibility of a two-way design, varying the levels of treatment A and treatment B.

Factorial clinical trials are experiments that test the effect of more than one treatment using a type of design that permits an assessment of potential interactions among the treatments.

In a factorial design there are two or more factors with multiple levels that are crossed, e.g., three dose levels of drug A and two levels of drug B can be crossed to yield a total of six treatment combinations:

low dose of A with low dose of B
low dose of A with high dose of B
mid dose of A with low dose of B
mid dose of A with high dose of B
high dose of A with low dose of B
high dose of A with high dose of B

Factorial designs offer certain advantages over conventional designs. There are a number of ways that you could look at these groups. This lesson will consider these alternatives...

Learning objectives & outcomes

Upon completion of this lesson, you should be able to do the following:

  • identify the conditions that would allow a factorial design to be useful
  • recognize the difference between qualitative and quantitative interactions
  • recognize the situation for which a ‘min’ test is the appropriate analysis

Reference

Piantadosi Steven. (2005) Reporting and Authorship. Factorial Designs. In: Piantadosi Steven. Clinical Trials: A Methodologic Perspective. 2nd ed. Hobaken, NJ: John Wiley and Sons, Inc.