Talk:System dynamics

A suggestion
Thanks for creating several animations. However, I think that the detailed and dynamic animations somewhat hinder the core concepts of system dynamics; stock and flow diagram. It would be great if various examples of simple stock and flow diagram are provided first and then show the the animations and simulations (e.g., the animations are moved at the end of article).

--Sangdon Lee (talk) 16:00, 6 June 2013 (UTC)
 * Hi Sangdon Lee, if you give me an example, I will try to do it.Patrhoue 17:00, 17 June 2013 (UTC) — Preceding unsigned comment added by Patrhoue (talk • contribs)

Hi Patrhoue
 * I sent an e-mail using "Email this user". I can send a Powerpoint file which contain various simple stock and flow diagrams.

--Sangdon Lee (talk) 15:42, 18 June 2013 (UTC)

Five important concepts in system dynamics
1. Stock and Flow (They are the same as integration and differentiation in calculus)

2. Circular causal feedback loops (positive and negative feedback loops)

3. Dynamic complexity (i.e., Things change and are connected)

4. Endogenous view (e.g. We met the enemy and he/she is us)

5. Time delays (The most disturbing factor in feedback system)

--Sangdon Lee (talk) 16:01, 6 June 2013 (UTC)

System Dynamics in one sentence: Stock (i.e, Accumulation) and Flow

 * X(t + DT) = X(t) + (inflow during DT - outflow during DT)*DT

e.g., the population in next year, X(t+DT), is equal to the population in this year, X(t), plus the net change during one year {i.e., difference between births and deaths during this year (i.e., DT)}.

e.g., the money in my bank this month is equal to the money in my bank last month plus the difference between deposits and widthdrawls during last month

Various names for stock and flow:


 * Stock is also known as level, accumulation, state variable in control theory, or integrals in calculus.
 * Flow is also called as rate, derivatives in calculus, &emsp; $$\frac{dx}{dt}$$, &emsp; $$\dot{x}$$, &emsp; $$\ddot{x}$$

--Sangdon Lee (talk) 16:01, 6 June 2013 (UTC)

The stock and flow diagram is the same as derivative and integral in calculus
X(t + ∆t) = X(t) + (inflow during ∆t - outflow during ∆t)*∆t

Move the X(t) to the left side:

X(t + ∆t) - X(t) = (inflow during ∆t - outflow during ∆t)*∆t

Divide the both side with ∆t:

{X(t + ∆t) - X(t)}/∆t = (inflow during ∆t - outflow during ∆t)

As the ∆t becomes close to zero (i.e, lim ∆t → 0), the above equation (i.e., System Dynamics in one sentence) becomes:

dX/dt = inflow - out flow = netflow

This is known as ordinary differencial equation (ODE).

e.g., $$\frac{dX}{dt} = a*X$$,

The stock and flow is another name of derivative and integral in calculus. The small difference in notation/concept between differentiation and stock and flow diagram (i.e., integration) in SD) makes a huge difference. It is much easier to understand the stock and flow diagram (i.e., bathtub analogy with inflow & outflow) than what the differential equation, dX/dt = aX, means.

--Sangdon Lee (talk) 16:24, 10 June 2013 (UTC)

Re Grey box completion and validation
“See also“ “Grey box completion and validation“ has been removed anonymously without explanation from this and several other topics. Following advice from Wikipedia if there are no objections (please provide your name and reasons), I plan to reinstate the reference in a weeks time. The removed reference adds additional information to the related fields subheading. In particular most models are incomplete (i.e. a grey box) and thus need completion and validation. This reference seems to be within the appropriate content of the “See also” section see Wikipedia:Manual_of_Style/Layout#See_also_section.

BillWhiten (talk) 05:26, 22 March 2015 (UTC)

Need to slow down diagram
The motion in the diagram in the lead section needs to be slowed down by a factor of 5 or 10. Right now it moves so fast that it's impossible to follow visually, rendering it useless. Loraof (talk) 18:16, 20 February 2017 (UTC)

The same holds for the diagram in the "Dynamic simulation results" section. Loraof (talk) 18:20, 20 February 2017 (UTC)