fractionate(design, fraction)
1/2
,
1/4
), or a formula
containing the terms to be used as defining contrasts.
fraction
.
The function only works for
2^k
designs.
Defining fractions for factors with three or more levels is more
complicated, and we have not attempted to do so.
The design has an attribute
"fraction"
containing the defining contrast(s).
If the
fraction
argument is numeric,
fractionate
will choose a fraction according to a
set of defining contrasts representing an
attempt to allow estimation of as many low-order effects as possible.
(The fractions are in the object
dimdc.list
if you want to look
at them.)
If a formula is supplied for
fraction
,
its terms should generally be simple high-order
interactions -- that is, factor names linked by
:
(colon). Each such
term defines one interaction (combination of factors).
The design will be divided in half according to whether the
corresponding contrast variable has value +1 or -1.
The term can appear in the formula with sign either "
+
" or "
-
", and
the positive or negative half will be chosen accordingly.
Notice that negative and positive terms do not cancel in this use of
formulas.
# a 1/4 replicate from the full 2^5 design in full.design # specifying the -1 fraction for the first contrast davies.design <- fractionate(full.design, fraction = ~ -A:C:D + A:B:E)