Fractional Factorial Designs using FrF2

May 18, 2011
By

(This article was first published on Software for Exploratory Data Analysis and Statistical Modelling, and kindly contributed to R-bloggers)

The FrF2 package for R can be used to create regular and non-regular Fractional Factorial 2-level designs. It is reasonably straightforward to use.

First step is to install the package then make it available for use in the current session:

require(FrF2)

A basic call to the main functino FrF2 specifies the number of runs in the fractional factorial design (which needs to be a multiple of 2) and the number of factors. For example a three factor design would have a total of eight runs if it was a full factorial but if we wanted to go with four runs then we can generate the design like this:

> FrF2(4, 3)
   A  B  C
1  1 -1 -1
2 -1  1 -1
3 -1 -1  1
4  1  1  1
class=design, type= FrF2

The default output labels the factors A, B, C and so on and the factor levels are -1 and +1 for the two levels of each factor. We can change the level names to low and high using the default.levels function argument:

> FrF2(4, 3, default.levels = c("low", "high"))
     A    B    C
1 high high high
2  low high  low
3 high  low  low
4  low  low high
class=design, type= FrF2

The factors can be specified as a list of names rather than the number of factors via the factor.names argument:

> FrF2(4, factor.names = c("One", "Two", "Three"),
  default.levels = c("low", "high"))
   One  Two Three
1  low high   low
2 high high  high
3  low  low  high
4 high  low   low
class=design, type= FrF2

These are the basics and there are other features for greater control over the confounding between factors and their interactions that is introduced by using a fractional factorial design.

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