Class 1: Feedback Loops & Introduction to Modeling

(Book sections 1.1 & 1.4)

LOs

• explain what positive and negative feedback loops are

• recognize positive and negative feedback loops from a given description

• identify the state variables (stocks), parameters and units of a model

• write models for simple systems using the basic template: rate of change = total inflow - total outflow

Positive feedback loops

variable reinforces each other, amplification of a change/process

when X increases, X LATER increases and vice versa

move in the same direction

threshold!

Negative feedback loops

variable self regulates

when X increases, X LATER decreases and vice versa

moves in opposite directions

threshold!

Tricks to Determining pos or neg

multiplication trick 

• - x - = +

• + x + = +

• - x + = -

no matter how many variables you have (3+)

trace variable in process: is it REINFORCING or BALANCING?

Basic Model

Rate of Change = inflow - outflow

X’(t) = c - kX(t)

state variable = changes with time, the STOCK

parameters = constants 

UNITS MATTER!!