Warnings
Not all combinations of inputs are compatible with each other. You may avoid
unnecessary work by reading the advice below. In case of violating the
restrictions below, the specification of estimable 2fis will take precedence over
other specifications (you will be warned, if this happens; on re-opening the dialog,
you will then have the chance to adjust the settings).
Blocked designs (number of blocks larger than 1 on the Base Settings tab)
and specification of an estimable model are not compatible
with each other in the current version of the software.
Thus, you have to decide on using blocks OR specifying estimable 2-factor interactions.
(Nevertheless, 2 or 4 blocks can be specified as detailed in the
section Brief statistical background below.
Generators or a specific design (radio buttons on the Base Settings tab)
can neither be specified together with estimable 2-factor interactions.
Finally, it is neither possible to simultaneously specify
hard-to-change factors on the Factor Details tab and
estimable 2-factor interactions.Brief statistical background
This tab allows to specify 2-factor interactions (2fis) that are of special
interest and must be estimable from the experiment. Per default, it is assumed
that the design must be at least of resolution IV.
There are two different approaches to estimability: If it can realistically be assumed that
all the 2fis that are not of special interest are negligible, it is sufficient
to request that the specified 2fis are clear of aliasing with EACH OTHER. Otherwise,
i.e. if one is not prepared to assume that all the unselected 2fis are negligible,
it is more appropriate to request that the specified 2fis are clear of aliasing
with ANY 2fis (both with each other and the other 2fis). Even under this approach, the
design need not be resolution V, since the unselected 2fis may be aliased
with each other.
If a design with certain interactions to be estimable is to be run in more
than four blocks, there is currently no automated way to produce it,
as it is not permitted to specify a number of blocks together
with selecting 2fis to be estimable. A blocked design with 2 or 4 blocks can be
obtained by explicitly including one (2 blocks) or two (4 blocks) blocking factors.
For the case of 4 blocks, it is important to include the interaction between
the two blocking factors into the selected interactions.
For the case of 2 blocks only, the blocking factor is treated like the
other factors, i.e. if its interactions with other factors are not among the selected ones,
they may be aliased with other 2fis (and even with main effects,
if resolution III is permitted in exceptional situations), which is usually
considered adequate.
The design is created with function FrF2 from package FrF2.
The special functionality for estimable 2fis is described here.
Users who are not familiar with the terminology around regular fractional factorial designs (e.g.
the concept of resolution) can browse the DoEGlossary.