Models in IE Lecture 13

System Dynamics

2nd Lecture

Systems Archetypes

Lecture 13: Outline• More examples of causal-loop diagrams

• Glucose and Insulin– What is wrong?– How could such a bad system evolve?

• Natural systems, man-made systems– What to look for when systems behave poorly

• Common Patterns: System Archetypes– Success to the successful– Escalation

Analysis of an argument

or

How to make a mountain out of a molehill

A’sanger

B’sharshness

A’s harshness

B’sanger

+

+

+

+

Mountains out of Molehills

• Whose fault is it?– “You started it”

• Cause, effect, and blame are not clear in complex systems

Eat Candy

BloodGlucose

Level

HungerLevel

+

-

+

Eat Candy

BloodGlucose

Level

HungerLevel

+

-

+

InsulinProduction

Delay

-+

0

5

10

15

20

25

30

35

40

45

Candy MoreCandy

Candy

GlucoseInsulin

Oscillations that increase in amplitude over time are

a danger signal

Glucose and Insulin

• How could nature be so stupid?

• How could such an unfit system survive?

Nature isn’t stupid -- We are!

• 1st appearance of insulin regulation– 10,000,000-70,000,000

years ago

• 1st appearance of refined sugar– 1500 to 2400 years ago

Eat Complex

Carbohydrates

BloodGlucose

Level

HungerLevel

+

-

+

InsulinProduction

Delay

-+

Delay

Eat Complex

Carbohydrates

BloodGlucose

Level

HungerLevel

+

-

+

InsulinProduction

Delay

-+

Delay

Eat Candy

+

+

+

Natural & Man-Made Systems

合Natural systems have usually evolved so that timings work well

› Introduce a new element into a smoothly running system and you are lucky if timings remain good› As change rate of technology this

problem occurs more frequently

“The road to hell is paved with good intentions”

“The hurrieder I go,

the behinder I get”

“The harder we run, the more weseem to stay in the same place”

“When your best isn’t good enough…”

“I keep banging my head

against the same wall”

Understanding System Performance

GoodB en ch m ark

Im p roveU n d ers tan d w h y

F o cu s o ncom p on en t(tes t b e fo rein ves tm en t)

UnsatisfactoryF a ilu re ap p aren t

in on ecom p on en t

L in kag esTim in g s

F eed b ack

UnsatisfactoryN o ap p aren t

fa ilu re incom p on en ts

AssessPerform ance

System Archetypes

• Recurring, classic patterns

• Identified in Peter Senge’s book, The Fifth Discipline (Chapters 6-16, Appendix 2)

• Should become part of your permanent vocabulary

• Escalation• Balancing loop with

delay• Success to the

successful• Tragedy of the

commons• Limits to growth• Shifting the burden

For Next Class…

• Read about the different system archetypes

• Bring your book to class– open book QuizQuiz forecast w.p. .8

Feedback: class performance on homework

• Good but not excellent overall

• Don’t complain about 6 hours/week!– 2nd half of term much harder

• Many of the harder problems are difficult conceptually, not technically– try harder– try more than once (many already do).

• Most work on Little’s Law problem, (6a.m, 6pm) was unsatisfactory.

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