the latter will be dummy factors; they will have no chemical
meaning at all. However it turns out that the apparent effects of
these dummy factors can be used to estimate the random
measurement errors (see below). The more dummy factors there
are, the better the estimate of such errors, so it is not
uncommon for experimenters to use a larger PB design than is
strictly necessary, thus getting higher quality information on
the signicance of each “real” factor.
PB designs utilise two levels for each factor, the higher level
being denoted “+” and the lower “” as usual. A further feature
of the PB method is that the + and signs for the individual
trial experiments are assigned in a cyclical manner. If we utilise
eight experiments with seven factors labelled A–G, the levels for
the rst experiment might be: