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Determining the weights of the attributes 41
According to this, the decision maker should choose office Y. This is
because it has the largest floor area and therefore performed best on
the attribute which was considered to be most important. However, a
look at the original figures reveals that Y’s floor area exceeds X’s by
only 2 square feet! Indeed, the weights imply that the decision maker
is prepared to sacrifice moving 15 miles closer to his customers to gain
just two extra square feet of office space, which seems an unlikely
proposition.
The problem with importance weights is that they may not take into
account the range between the least- and most-preferred options on each
attribute (see von Winterfeldt and Edwards 10 ). If the options perform
very similarly on a particular attribute, so that the range between worst
and best is small, then this attribute is unlikely to be important in
the decision, even though the decision maker may consider it to be an
important attribute per se. Taking this to extremes, suppose that both
offices had a floor area of 400 square feet. In this case, the weight attached
to floor area should be zero because this attribute has no importance in
discriminating between the two offices.
Fortunately, this problem can be avoided by using swing weights.
These are derived by asking the decision maker to compare a change
(or swing) from the least-preferred to the most-preferred value on one
attribute to a similar change in another attribute. The simplest approach
is to proceed as follows. Consider the lowest-level attributes on the
‘Benefits’ branch of the value tree (Figure 3.1). The owner is asked to
imagine a hypothetical office with all these attributes at their leastpreferred
levels, that is, an office which is the greatest distance (i.e.
8 miles) from the town center, has the worst position for visibility and
has the worst image, the smallest size and so on. Then he is asked, if just
one of these attributes could be moved to its best level, which would
he choose? The owner selects ‘closeness to customers’. After this change
has been made, he is asked which attribute he would next choose to
move to its best level, and so on until all the attributes have been ranked.
The owner’s rankings are:
(1) Closeness to customers
(2) Visibility
(3) Image
(4) Size
(5) Comfort
(6) Car-parking facilities