There is an ongoing discussion whether it is wise to randomize the run
order of a factorial experiment if there is concern about a possible time trend in the
experiment. It can be argued that a randomized order is not very effective because
the trend inflates the error. Some authors even criticize that a randomized order
will normally not be orthogonal to trend, they claim that therefore there will be bias
under the randomized order. On the other hand, a systematic order will only be
useful if the true trend is behaving as is predicted by the model.
The present paper investigates the properties of different run order strategies in a
simulation study with unreplicated factorial designs. We check to which extend the
presence of a time trend might inflate the probability of false rejection of a true nullhypothesis,
and we compare the power of significance tests based on the half-normal
plot under the various run order concepts
