“Good experimental design usually requires that we change only one factor at a time” according to an article I read recently in The Scientist magazine (“Why Don’t We Share Data,” page 33, Issue 4, Volume 23).  This guide for science fairs tells students that “you conduct a fair test by making sure that you change only one factor at a time while keeping all other conditions the same.” 

Obviously changing two variables together makes no sense, such as the time that as science project one of my kids asked me to do a blind taste test on Coke versus Pepsi, but to keep them straight in their mind, she poured one cola in blue plastic cup and the other in white Styrofoam!  Needless to say I was completely confounded.

The OFAT method is so engrained that it’s literally become the law according to scientist who told me that, when as an expert witness he presented statistically significant evidence, it was thrown out of court due to the experiment design having changed multiple factors simultaneously.  What a crime!

Multifactor testing is far more effective for statistical power, screening efficiency and detection of interactions.  Industrial experimenters are well-advised to forget their indoctrination in OFAT and make use of multifactorial designs.  For reasons why, see my two-part series on Trimming the FAT out of Experimental Methods and No-FAT Multifactor Design of Experiments.

Good experimental design does NOT require changing only one factor at a time!