Feedback Loops and Modeling

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 Loop (+)

• moving in the same direction

• A positive value of a variable leads to an increase in the variable

• A negative value of a variable leads to a decrease in that variable

• “reinforcing” feedback (accelerates/encourages the change in the initial variable + not always a “good” outcome)

Negative Feedback Loops (-)

• moving in the opposite direction

• A pos. value of a variable leads to a decrease in the variable

• A neg. value of a variable leads to an increase in the variable

• “initial increase leads to a later decrease,” vice versa.

• “maintains a variable around a specific threshold”

Description on feedback

Structure of a Model

• x(t) is a state variable

  • changes with time

• basic template for change equations: x’(t)=c-kX(t)

  • c is the inflow (does not change)

  • k is part of the outflow as a rate proportional to the variable (does not change = constant)

    • Both c and k are then parameters

• overall:

  • X’(t) = inflows - outflows

  • Units must be consistent