What is a System Model?
What can we use a system model for?
- A model that looks at the populations as a whole instead of individuals
- Interactions are represented between groups rather than individuals
- Looks at the flow of populations from one group to another
- After we build a model that applies what we understand about how a
system works, we can compare the behavior of the model to the
behavior of the actual system to see if our understanding of the
system is sufficient to account for it's behavior.
What is Vensim?
- Vensim is a tool for building systems models. The computer does the hard work and repeated calculations for you. The human does the thinking and planning by building the model. You are telling a story, and then the computer does the legwork of calculating the outcome of the story.
Talk about the spread of a disease
- Write this sentence on the board: "When a healthy person meets a sick person, there is a chance that the healthy person may become sick."
- It is worth mentioning that we are interested in the processes that
lead to changes in our populations of healthy and sick people.
"Gets sick" moves people from healthy to sick, so we need a
process called "gets sick" that connects these two boxes.
"Meets" is different, meeting doesn't cause people to move between
populations, but people have to meet for "get sick" to happen. So "meets" is a condition for "get sick".
- What are the nouns?
- Healthy People and Sick People (susceptible and infected)
- Draw two boxes on the board and label them Susceptible and Infected. Explain that these boxes represent the number of people in each group at any given time.
- What are the verbs?
- Become Sick (this is a verb that we want to model)
- Draw an arrow from Healthy People to Infected People and label it Get Sick (see the tutorial for the correct arrow style). This arrow represents the transition of healthy people into the sick group.
- What are the conditions?
- "Meets" is a condition for getting sick.
can represent meeting in the model by drawing arrows from each of
our populations to the "Get Sick" process.
- There is a "chance" that a healthy person will get sick
- Write "Infection fraction" to represent the chance that the disease will spread and point an arrow from the fraction to "Get Sick" because this fraction affects how many people get sick.
Physical Modeling and Measurement
No physical modeling in this lesson.
Build the model in Vensim
- Use the tutorial to build the model step-by-step
- If we are using the model in a scientific mode, to understand how a
real observable system works, then next step would be to compare
the model behavior to the behavior of the system. We can set or
control various properties in the model, and in an experiment on the
real system to see how well our model represents the system.
- If we are using the model to design something new, the next step is
to ask: Does it do what I need it to do? Then set about adjusting the
model until the answer is "yes".