Add Heterogeneity

One thing that sets agent-based modeling apart from analytic techniques or system dynamic models is the ease with which you can add 'heterogeneity'. This means that our population of agents can have different demographic features; for example in HASH you can easily create a distribution of age by adding a property and assigning different age values to different agents.

These different values can allow our agents to all behave in slightly different ways, or cause other agents to interact with them differently.

In our Getting Started model (Python or Javascript), we'll start by providing our agents with an 'at_risk' property. Different people agents will have different chances of having a severe response to getting sick. In the "create_people" behavior, new agents are defined in the person variable. In that block of code, add a line for the property "at_risk":

// line 26
agents["people"].push({
    // ...
    at_risk: Math.random() < 0.5 ? true : false
}

In the "infection" behavior, when a Person agent gets infected, let's add logic to determine the severity of the infection:

// infection.js
// line 81
const severe_chance = state.at_risk
  ? at_risk_chance_of_severe
  : chance_of_severe;

if (state.severity === "moderate" && Math.random() < severe_chance) {
  state.severity = "severe";
}

When we instantiate a Person currently, they'll have a 50% chance of being at_risk from the virus, and if they're infected, they'll have a different likelihoods of getting a severe infection. These likelihoods are defined in the "globals.json" file.

Of course during different scenarios and experiments we might want to vary the likelihood that someone will be at risk. Instead of hardcoding the likelihood, we will add a property called at_risk_percent to "globals.json". Here we set it to 5%:

globals.json

{
"at_risk_percent": 0.05
}

And in the "create_people" behavior, we substitute that value for the hardcoded one:

// line 26
agents["people"].push({
    // ...
    at_risk: Math.random() < context.globals().at_risk_percent ? true : false,
}