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Samenvatting Thinking in Systems
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- Thinking in Systems
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[Meer zien]Voorbeeld 3 van de 18 pagina's
Voorbeeld 3 van de 18 pagina's
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Summary Thinking in SystemsChapter 1 The basics
A system is an interconnected set of elements that is coherently organized in a way that achieves
something. It must consist of three kinds of things: elements, interconnection, and a function or
purpose. Systems can be embedded in systems, which are embedded in yet other systems. There are
also things that are not a system such as a conglomeration without any particular interconnections or
function.
Look beyond the players of the rule of the game you think that because you understand “one” that
you must therefore understand “two” because one and one makes two. But you forget that you must
also understand “and”.
The elements of a system are often the easiest parts to notice, because many of them are visible,
tangible things. Elements do not have to be physical things. Intangibles are also elements of a
system. You can divide the elements into sub-elements and sub-sub-elements. Before going to far in
that direction, it is a good idea to stop dissecting out elements and to start looking for the
interconnections, the relationships that hold the elements together. Some interconnections in
systems are actual physical flows. Many interconnections are flows of information – signals that go to
decision points or action points within a system. They are harder to see, but the systems reveals
them to those who look. Information holds systems together and plays a great role in determining
how they operate.
If information-based relationships are hard to see, functions or purpose are even harder. A system’s
function or purpose is not necessarily spoken, written or expressed explicitly, except through the
operation of the system. The best way to deduce the system’s purpose is to watch for a while to see
how the system behaves. Purpose are deduced from behaviour, not rhetoric or stated goals.
The word function is generally used for a nonhuman system, the word purpose for a human one, but
the distinction is not absolute, since so many systems have both human and nonhuman elements.
System purposed need to be human processed and are not necessarily those intended by any single
actor within a system. In fact, one of the most frustrating aspects of systems is that the purposes of
subunits may add up to an overall behaviour no one wants. Systems can be nested within systems.
Therefore, there can be purposes within purposes.
You can understand the relative importance of a system’s elements, interconnections and purpose
by imagining them changed one by one. Changing elements usually has the least effect on the
system. A system generally goes on being itself, changing only slowly if at all, even with complete
substitution of its elements – as long as its interconnections and purpose remain intact. If the
interconnections change, the system may be greatly altered. It may even become unrecognizable.
Changing interconnections in a system can change it dramatically. Changes in function or purpose
can also be drastic. A change in purpose changes a system profoundly, even if every element and
interconnection remains the same.
To ask whether elements, interconnection or purpose are most important in a system is to ask an
unsystemic question. All are essential. The least obvious part of the system, its function or purpose, is
often the most crucial determinant of the system’s behaviour. Interconnections are also critically
important. Changing relationships usually changes system behaviour. The elements, the part of
systems we are most likely to notice, are often (not always) least important in defining the unique
,characteristics of the system – unless changing an element also results in changing relationships or
purpose.
A stock is the foundation of any system. Stocks are the elements of the system that you can see, feel,
count, or measure at any given time. A system stock is just what is sounds like: a store, quantity,
accumulation of material or information that has built up over time. A stock does not have to be
physical. Stocks change over time through the actions of a flow. Flows are filling and draining, births
and deaths, purchases and sales, growth and decay, deposits and withdrawals, successes and
failures. A stock then it he present memory of the history of changing flows within that system.
Figure 1 how to read stock-and-flow diagrams. In this book, stocks are shown as boxes, and flows are arrow-headed “pipes”
leading into or out of our stocks. The “clouds” stand for wherever the flows come form and go to – the sources and sinks that
are being ignored for the purposes of the present discussion.
If you understand the dynamics of stocks and flows – their behaviour over time – you understand a
good deal about the behaviour of complex systems.
System thinkers use graphs of system behaviour to
understand trend overtime, rather than focussing
attention on individual events. We also use behaviour-
over-time graphs to learn whether the system is
approaching a goal or a limit, and if so, how quickly. The
variable on the graph may be a stock or a flow. The
pattern – the shape of the variable line – is important,
as are the points at which that line changes shape or
direction. The precise number on the axes are often less
important. The horizontal axes of time allows you to ask questions about what came before, and
what might happen next. It can help you focus on the time horizon appropriate of the question or
problem you are investigating.
All models whether mental models or mathematical models, are simplifications of the real world.
Several important principles that extend to more complicated systems:
As long as the sum of all inflows exceed the sum of all outflows, the level of stock will rise
As long as the sum of all outflows exceed the sum of all inflows, the level of stock will fall
If the sum of all outflows equals the sum of all inflows, the stock level will not change; it will
be held in dynamic equilibrium at whatever level it happened to be when the two sets of
flow became equal.
The human mind seems to focus more easily on stocks than flows. On top of that, when we do focus
on flows, we tend to focus on inflows more easily than outflows. A stock can be increased by
decreasing its outflow rate as well as by increasing its inflow rate. A stock takes time to change,
because flow takes time to flow. That’s a vital point, a key to understands why systems behave as the
do. Stocks usually change slow. They can act as delays, lags, buffers, ballast, and source of
momentum in a system. Changes in stocks sets the pace of the dynamics of the system.
, Industrialization cannot proceed faster than the rate at which factories and machines can be
constructed and the rate at which humans can be educated to run and maintain them.
The time lags that come from slowly changing stocks can cause problems in systems, but they also
can be sources of stability. The time lags imposed by stocks allow room to manoeuvre, to
experiment, and to revise policies that are not working.
There is one more important principle about the role of stocks in systems, a principle that will lead us
directly to the concept of feedback.. the presence of stock allows inflows and outflows to be
independent of each other and temporarily out of balance with each other. People monitor stocks
constantly and make decisions and take action designed to raise or lower stocks tor to keep them
within acceptable ranges. Those decisions add up to the ebbs and flows, success and problems of all
sorts of systems. System thinkers see the world as a collection of stocks along with the mechanisms
for regulating the levels in the stocks by manipulating flows. That means system thinkers see the
world as a collection of “feedback processes”.
The mechanism operates through a feedback loop. It is the consistent behaviour pattern over a long
period of time that is the first hint of existence of a feedback loop. A feedback loop is formed when
changes in a stock affect the flows into or out of that same stock. A feedback loop can be quite
simple and direct. Feedback loops can cause stock to maintain their level within a range or grow or
decline. In any case, the flows into or out of the stock are adjusted because of changes in the seize of
the stock itself. The stock level feeds back through a chain of signals and cations to control itself. A
feedback loop is a closed chain of causal connections from a stock, through a set of rules or physical
laws or actions that are dependent on the level of the stock, and back again through a flow to change
the stock.
Figure 2 how to read a stock-and-flow diagram with feedback loops. Each diagram distinguishes the stock, the flow that
changes the stock, and the information link (shown as a thin curved arrow) that directs the action.
A reinforcing loop. B balancing loop and delay.
Not all systems have feedback loops. Some systems are relatively open-ended chains of stocks and
flows. The chain may be affected by outside factors, but the level of the chain’s stock don’t affect it
flows.
One common kind of feedback loop stabilizes the stock level. The stock level may not remain
completely fixed, but it does stay within an acceptable range. What follows are some more stabilizing
feedback loops. Balancing feedback loops are goal-seeking or stability-seeking. A balancing feedback
loop opposes whatever direction of change is imposed on the system. If you push a stock too far up,
balancing loop will try to pull it back down and the other way around. Balancing feedback loops are
equilibrating or goal-seeking structures in systems and are both sources of stability and sources of
resistance to change.
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