Stocks and flows and word-and-arrow diagrams
Posted: Thu Apr 17, 1997 1:57 pm
We seem to be getting ourselves in knots over the question of whether to
use stocks and flows ("tubs and pipes with valves") in our systems
diagrams or whether to limit ourselves to words and arrows without
explicit recognition of stocks and flows.
Cant we treat this as we treat a number of other issues, by agreeing to
fit "best" practice to audience and purpose and the nature of the system
under discussion? The purpose of our diagrams is always to communicate --
usually to others, but sometimes to ourselves as we immerse ourselves in
the conceptualization and formulation of a model. If stocks and flows
enhance communication, we should represent them in our models, both in
the conceptualization stage or the reporting phase. If stocks and flows
dont help the communication or understanding, which sometimes happens,
then we should feel ok to not represent them in our diagrams.
Examples: 1) The flow of individuals from one accumulation to another
(e.g., new workforce to experienced workforce, work in process to finished
inventory, susceptibles to the sick population) is difficult to see in
causal-loop form and easy to see in stocks and flows. [In the causal-loop
form, the single flow from stock A to stock B has to be represented as two
arrows, one subtracting from A and another adding to B, thus obscuring
that there is one flow between the two stocks.] So in such a chain of
accumulations one should almost always use explicit stocks and flows,
no matter what the audience, since everyone will find the stock-and-flow
picture to be the clearest representation.
2) In a recent group model conceptualization process, we were talking
about communication and trust in a working group and the resulting
dynamics of effectiveness of teams and team learning. The stocks were
very unclear to the group at the outset, so it seemed most appropropriate
to begin the conceptualization using nothing but words and arrows (causal
loops, influence diagrams). A clear stock emerged at one point in the
distinction between "Teamwork" and "Experience with Teamwork" -- the group
saw the latter as a stock of experience whose rate of growth depended on
the amount of teamwork present at any given time. Trust later seemed to
have the character of a stock, so we identified it as such later. We made
much headway by not restricting ourselves to identifying stocks and flows
at the outset.
So let us treat the stock-and-flow / word-and-arrow issue as a judgment
call a professional makes as he or she is developing views of system
structure. Use the representation that is most helpful at the time.
Highlighting clear stocks is usually helpful, so there ought to be a bias
to representing clear stocks and flows where they clearly exist. But the
judgment also must include the characteristics of the audience one is
communicating with and the purpose of the diagrams.
I guess my conclusion is
form of representation = f ( audience, purpose, system )
Im hoping this apparent equivocation is satisfying to all, including
those of us who have specific biases, and is scientifically respectable as
well. It does seem to reflect "best practice."
...GPR
-----------------------------------------------------------------------
George P. Richardson G.P.Richardson@Albany.edu Rockefeller College of
Public Affairs and Policy Phone: 518-442-3859 University at Albany - SUNY,
Albany, NY 12222 Fax: 518-442-3398
-----------------------------------------------------------------------
use stocks and flows ("tubs and pipes with valves") in our systems
diagrams or whether to limit ourselves to words and arrows without
explicit recognition of stocks and flows.
Cant we treat this as we treat a number of other issues, by agreeing to
fit "best" practice to audience and purpose and the nature of the system
under discussion? The purpose of our diagrams is always to communicate --
usually to others, but sometimes to ourselves as we immerse ourselves in
the conceptualization and formulation of a model. If stocks and flows
enhance communication, we should represent them in our models, both in
the conceptualization stage or the reporting phase. If stocks and flows
dont help the communication or understanding, which sometimes happens,
then we should feel ok to not represent them in our diagrams.
Examples: 1) The flow of individuals from one accumulation to another
(e.g., new workforce to experienced workforce, work in process to finished
inventory, susceptibles to the sick population) is difficult to see in
causal-loop form and easy to see in stocks and flows. [In the causal-loop
form, the single flow from stock A to stock B has to be represented as two
arrows, one subtracting from A and another adding to B, thus obscuring
that there is one flow between the two stocks.] So in such a chain of
accumulations one should almost always use explicit stocks and flows,
no matter what the audience, since everyone will find the stock-and-flow
picture to be the clearest representation.
2) In a recent group model conceptualization process, we were talking
about communication and trust in a working group and the resulting
dynamics of effectiveness of teams and team learning. The stocks were
very unclear to the group at the outset, so it seemed most appropropriate
to begin the conceptualization using nothing but words and arrows (causal
loops, influence diagrams). A clear stock emerged at one point in the
distinction between "Teamwork" and "Experience with Teamwork" -- the group
saw the latter as a stock of experience whose rate of growth depended on
the amount of teamwork present at any given time. Trust later seemed to
have the character of a stock, so we identified it as such later. We made
much headway by not restricting ourselves to identifying stocks and flows
at the outset.
So let us treat the stock-and-flow / word-and-arrow issue as a judgment
call a professional makes as he or she is developing views of system
structure. Use the representation that is most helpful at the time.
Highlighting clear stocks is usually helpful, so there ought to be a bias
to representing clear stocks and flows where they clearly exist. But the
judgment also must include the characteristics of the audience one is
communicating with and the purpose of the diagrams.
I guess my conclusion is
form of representation = f ( audience, purpose, system )
Im hoping this apparent equivocation is satisfying to all, including
those of us who have specific biases, and is scientifically respectable as
well. It does seem to reflect "best practice."
...GPR
-----------------------------------------------------------------------
George P. Richardson G.P.Richardson@Albany.edu Rockefeller College of
Public Affairs and Policy Phone: 518-442-3859 University at Albany - SUNY,
Albany, NY 12222 Fax: 518-442-3398
-----------------------------------------------------------------------