Soft variables

[jforestr@MIT.EDU (Jay W. Forrest]

On Dec. 18, Stephen Robbins asked for references on soft variables.
There may not be a full specific discussion of the subject, but various
insights have been discussed in some of my writing. I will give the
references. However, there is no sharp distinction between hard and
soft variables. Take an inventory for example, inventory should be one
of the "harder" variables, but what per cent of orders can be filled
from a given inventory level? That is an estimate based on experience
and on interpretations of insufficient data. Likewise, the decision
statement about how much to reorder for inventory depends on rather soft
concepts about the extent to which the person ordering takes into
account delivery delay, goods already on order, trends in sales, etc.
There is a continuum from hard variables, which usually are not very
hard, to those that are very, very soft.

The following references contain related material, with a fair amount of
repetition among them:

Forrester, Jay W. (1992). Policies, Decisions, and Information Sources
for Modeling. European Journal of Operational Research, 59(1), pp.
42-63.

The above is reprinted in:
Forrester, Jay W. 1994. "Policies, Decisions, and Information Sources
for Modeling." In John D. W. Morecroft, & John D. Sterman (ed.),
Modeling for Learning Organizations. pp. 51-84. Portland, OR:
Productivity Press.

Forrester, Jay W. 1993. "System Dynamics and the Lessons of 35 Years."
In Kenyon B. De Greene (ed.), Systems-Based Approach to Policymaking.
pp. 199-240. Norwell, MA: Kluwer Academic Publishers.

Forrester, Jay W. (1980). Information Sources for Modeling the National
Economy. Journal of the American Statistical Association, 75(371), pp.
555-574.

Forrester, Jay W., and Peter M. Senge. 1980. "Tests for Building
Confidence in System Dynamics Models." In A. A. Legasto Jr. , J. W.
Forrester, & J. M. Lyneis (ed.), System Dynamics. Series: TIMS Studies
in the Management Sciences. pp. 209-228. New York: North-Holland.

Especially Part II in the following:
Forrester, Jay W. (1961). Industrial Dynamics. Portland, OR:
Productivity Press. 464 pp.

The following two books contain detailed models with many
soft-variable concepts:

Forrester, Jay W. (1969). Urban Dynamics. Portland, OR: Productivity
Press. 285 pp.

Forrester, Jay W. (1973). World Dynamics. (2 ed.). Portland, OR:
Productivity Press. 144 pp.