# fracfactgen

Fractional factorial design generators

## Syntax

`generators = fracfactgen(terms)`

generators = fracfactgen(terms,k)

generators = fracfactgen(terms,k,R)

generators = fracfactgen(terms,k,R,basic)

## Description

`generators = fracfactgen(terms)`

uses the
Franklin-Bailey algorithm to find generators for the smallest two-level
fractional-factorial design for estimating linear model terms specified by
`terms`

. `terms`

is a character vector or string
scalar consisting of words formed from the `52`

case-sensitive letters
`a`

-`Z`

, separated by spaces. Use
`'a'-'z'`

for the first 26 factors, and, if necessary,
`'A'-'Z'`

for the remaining factors. For example, ```
terms =
'a b c ab ac'
```

. Single-letter words indicate main effects to be estimated;
multiple-letter words indicate interactions. Alternatively, `terms`

is
an *m*-by-*n* matrix of `0`

s and
`1`

s where *m* is the number of model terms to be
estimated and *n* is the number of factors. For example, if
`terms`

contains rows `[0 1 0 0]`

and ```
[1
0 0 1]
```

, then the factor `b`

and the interaction between
factors `a`

and `d`

are included in the model.
`generators`

is a cell array of character vectors with one
generator per cell. Pass `generators`

to `fracfact`

to produce the fractional-factorial design and corresponding
confounding pattern.

`generators = fracfactgen(terms,k)`

returns
generators for a two-level fractional-factorial design with 2

-runs,
if possible. If ^{k}`k`

is `[]`

, `fracfactgen`

finds
the smallest design.

`generators = fracfactgen(terms,k,R)`

finds
a design with resolution `R`

, if possible. The default
resolution is `3`

.

A design of *resolution* *R* is
one in which no *n*-factor interaction is confounded
with any other effect containing less than *R* – *n* factors.
Thus a resolution III design does not confound main effects with one
another but may confound them with two-way interactions, while a resolution
IV design does not confound either main effects or two-way interactions
but may confound two-way interactions with each other.

If `fracfactgen`

is unable to find a design
at the requested resolution, it tries to find a lower-resolution design
sufficient to calibrate the model. If it is successful, it returns
the generators for the lower-resolution design along with a warning.
If it fails, it returns an error.

`generators = fracfactgen(terms,k,R,basic)`

also
accepts a vector `basic`

specifying the indices of
factors that are to be treated as basic. These factors receive full-factorial
treatments in the design. The default includes factors that are part
of the highest-order interaction in `terms`

.

## Examples

Suppose you wish to determine the effects of four two-level
factors, for which there may be two-way interactions. A full-factorial
design would require 2^{4} = 16 runs. The `fracfactgen`

function
finds generators for a resolution IV (separating main effects) fractional-factorial
design that requires only 2^{3} = 8 runs:

generators = fracfactgen('a b c d',3,4) generators = 'a' 'b' 'c' 'abc'

The more economical design and the corresponding confounding
pattern are returned by `fracfact`

:

[dfF,confounding] = fracfact(generators) dfF = -1 -1 -1 -1 -1 -1 1 1 -1 1 -1 1 -1 1 1 -1 1 -1 -1 1 1 -1 1 -1 1 1 -1 -1 1 1 1 1 confounding = 'Term' 'Generator' 'Confounding' 'X1' 'a' 'X1' 'X2' 'b' 'X2' 'X3' 'c' 'X3' 'X4' 'abc' 'X4' 'X1*X2' 'ab' 'X1*X2 + X3*X4' 'X1*X3' 'ac' 'X1*X3 + X2*X4' 'X1*X4' 'bc' 'X1*X4 + X2*X3' 'X2*X3' 'bc' 'X1*X4 + X2*X3' 'X2*X4' 'ac' 'X1*X3 + X2*X4' 'X3*X4' 'ab' 'X1*X2 + X3*X4'

The confounding pattern shows, for example, that the two-way
interaction between `X1`

and `X2`

is
confounded by the two-way interaction between `X3`

and `X4`

.

## References

[1] Box, G. E. P., W. G. Hunter, and J.
S. Hunter. *Statistics for Experimenters*. Hoboken,
NJ: Wiley-Interscience, 1978.

## Version History

**Introduced in R2006a**