Design of Experiments - US Army Conference on Applied Statistics

Why isn’t orthogonality a design criterion?
1. Not all orthogonal designs are good.
a) It is inappropriate to change the requirements <strong>of</strong> a problem to use
an orthogonal design as a “solution”.
b) As we will see, in many practical situations no orthogonal design
exists.
2. Not all good designs are orthogonal.
Sometimes it may be useful to sacrifice orthogonality for some other
desirable design feature.
3. In standard two-level screening design orthogonal designs
minimize the variance <strong>of</strong> the coefficient estimates, so
focusing on variance results in orthogonal designs, if they
are possible.