Kym Warren and Bruce Campbell make some interesting points in this
discussion of the two-analyst problem - two equivalent people should come
up with something like the same model given the same information. Ideally
that should be the case though we need to remember that even eminent
surgeons dont practise medicine identically and might disagree on their
diagnoses, though one would hope (perhaps vainly) for some convergence
through their ability to share their discipline. Similarly, opposing lawyers
will differ in their interpretation of the law, and two engineers might
propose different designs for a given bridge. The two analyst problem has
thus not been completely solved even in disciplines supported by empirical
research (medicine and engineering) or centuries of profound scholarship on
established principles of justice.
Oddly, SD might be in a stronger position - Ill try to explain why I think
that and see if I get torn to shreds.
Classical SD suggests that one starts with a reference mode of historical
behaviour to be improved on or of future behaviour to be achieved. One then
selects some suitably important level variables (why do we now call these
stocks?) and models the causal forces which those levels produce and which,
in turn, cause their dynamics. Clearly if I select one set of levels and you
select another we are likely to have two radically different models and the
process will be subjective. There is also a risk of confusing two meanings
of importance. Does it mean that the variable is important in producing
observed behaviour or does it mean important to me in that behaviour?
Bruce suggests that behaviour mimicking the real system is an important
determinant of validity, but that can be achieved to arbitrary accuracy by a
suitable collection of Fourier transforms which has no causal content and
hence no policy implications. I phrase it as does the model do the same
things as the real world AND FOR THE SAME REASONS? - within the limits of
the simplifications I have made. The simplifications are an important
consideration as there is otherwise a tendency to make the model more and
more and more detailed in a vain search for an illusion of accuracy.
Unhappily, I have seen many models which are so large that they have ceased
to be models and have become large computer programs. Since error is
inevitable in any very large program, even to the extent of typing * when /
was needed, the outputs from a large program can be so unreliable that the
policy conclusions may be misleading or downright wrong.
There are, in my experience, four guidelines for avoiding the subjectivity
trap.
FIRST, to try to keep models reasonably small so that they can be discussed
and checked. The documentor facility in DYNAMO (and in DYSMAP and COSMIC)
was truly excellent for that. It made it easy to study the equations of a
model, something which it is now fairly rare to see people doing - though I
do see people staring at a stock/flow diagram (or the fragment which can be
seen on the screen) in the mistaken belief that it is the model.
SECOND, the emphasis on physical flow is vital and is the theoretical basis
of SD. We all know that, of course, except that (and you may not believe
this) I have met people who have never heard of that idea and yet claim to
practice system dynamics! Depressing, isnt it?
THIRD, and following from that, is the necessity for conservation of mass
and hence the cardinal importance of dimensional validity. I have seen
numerous models which are, at a glance, wildly dimensionally invalid and
hence are quite untrustworthy. Why is that? Simply because I know
practitioners and academics who have never heard of dimensional validity.
FOUR, then, is the need for practitioners, whether novices or virtuosi,
actually to read the texts on SD and not to rely on software manuals.
Excellent though those are, their purpose is to explain the use of a
computer program and teaching the precepts and practices of a discipline is
something different. Even after 30+ years in SD I still dip in to Jays
Industrial Dynamics from time to time and always to my profit. (Hands up all
those who have never read it and both hands up all those who have never
heard of it. I wish we could see the response to that - it might be very
depressing).
By the way, and if I dare mention it, my book actually refers to the
two-analyst problem (page 33) and states that it is one of the attractive
features of list extension that two analysts tackling the same problem
should come up with recognisably similar solutions (i.e.models) even if they
started with different ideas as to what were the key variables. (Hands up
those who have never heard of that.)
What an interesting discussion. It makes a welcome change from people asking
if there is a model they can use for such and such a problem. Models are
built to answer questions (see Richardsons and Pughs book - hands up all
those who have never read it) and someone elses questions are not likely to
be the same as yours.
What Kym and Bruce have put their expert fingers on is probably the need
actually to learn something about SD before rushing off and doing it.
Regards,
Geoff
geoff.coyle@btinternet.com
Professor Geoff Coyle
Consultant in System Dynamics and Strategic Analysis
Tel: (44) 01793 782817 Fax: 01793 783188