Some would say not , arguing that the whole is more than and more significant
than the sum of its component parts; that ability to avoid even a single error
during a complex process is the essence of competence. The argument is not
without merit. Surely it is true that in the ordinary affairs of living , single errors
can be very costly . One thing done wrong can cancel the rewards for doing many
things right. But is our purpose in measuring mental abi lities to imitate life? Or is
it mainly to assess a person's cognitive resources, that is, the person's knowledge
and mental abi lities? For that purpose it may be appropriate and advantageous to
take each decision into account and to assess them independently. It may be
inappropriate and disadvantageous to consider only a single outcome from a
sequence or cluster of related , contributory decisions.