Software and R packages installation and working directory setup.

Please make sure you downloaded the following software and R packages.

Tips for using the exercise notebook.

The files we will need for this exercise are listed below. All files can be downloaded here (some are in the folder “AdditionalExercises”).

File name Content
itims_workshop.Rmd The exercise note in R markdown
itims_workshop.html The html version of the exercise note (can be viewed in web browsers)
model_shiny.Rmd shiny interface for the models we use in the exercise
empty_example_model.nlogo the empty example model used in Problem 1C
SIR-agent-based.nlogo An agent based model of disease transmission in NetLogo
SIR-deterministic.nlogo A deterministic population model of disease transmission in NetLogo
SIR-stochastic.nlogo A stochastic population model of disease transmission in NetLogo
turtle_color.nlogo An example model from optional exercises

This notebook is written in R Markdown (the .Rmd file). This generates the HTML version of the notebook (the .html file), which can be viewed in your browser. To run the R code in the notebook:

  1. You can copy the code in R code chunks from the .html file (you can also copy from the .Rmd if you prefer), paste it into the RStudio or R console, and hit “enter”.

  2. If you don’t want to have to keep copy-pasting into the console and you are using RStudio, you can also make a new script file and paste the R code chunks into that—then you can execute those lines of code by highlighting them and then clicking the drop-down arrow of Run button and then Run Selected Line(s), or just hit ctrl-enter (windows) or cmd-return (mac) to run the highlighted code.

  3. Alternatively, if you are using RStudio (all versions), you can highlight the code you want to excute in the .Rmd file and then click the drop-down arrow of Run button and click Run Selected Line(s). You can also just hit ctrl-enter (windows) or cmd-return (mac) to run the highlighted code.

  4. If your version of RSutdio is 1.0 or higher (RStudio > About RStudio), you can also click the right arrow at the upper right hand corner of a R code chunk to excute the chunk. You will see the outputs immediately after the R code chunck. In order to see the outputs in the Viewer or Plots window, click the gear icon Smiley face and then check Chunk Output in Console.

Note1: in order to create a script to save the R code you work on in RStudio, you can click the New File icon Smiley face and then select R Script. To run the whole script, just click the Run button.

Note2: the itims_workshop.html can be generated from itims_workshop.Rmd by clicking Smiley face

Recording your answers and observations

If you want to record your answers to the problems as you go, the easiest way is probably to use this form (plus, that way we can see how people are progressing and what you all discover!). But if you’d rather not, feel free to write your answers on paper or in the text boxes below. Just be careful—answers in the text boxes below will not be saved if you reload the page!



Problem 1A: Simple rules can lead to complex behavior.

During the 1960s the economist Thomas Schelling researched neighborhood racial segregation and residential preferences. Using a simple model, he showed that individuals can have only mild preferences for the same type and yet these subtle preferences can lead to societal segregation. Here, we’ll explore this with a hypothetical setting, looking at interactions between red and green turtles.

In this model, the red turtles and green turtles get along with one another. But each turtle wants to make sure that it lives near some of “its own.” That is, each red turtle wants to live next to at least some red turtles, and each green turtle wants to live next to at least some green turtles.

So, each turtle is happy if at least a certain percentage of their neighbors are the same type as them (this percent is given by the parameter %-similar-wanted in the NetLogo file). If a turtle is happy, they stay in their current location. If they are unhappy, they move to a randomly chosen new location. This continues until all the people are happy (or possibly forever).




Problem 1B: Disease spread on networks.

Next, we’ll look at how disease spreads on contact networks between individuals. Open the Virus on a Network model, in File > Models Library > Sample Models > Networks > Virus on a Network. This model simulates disease spread model on a network, where infected individuals can either return to the susceptible class or become immune. Simulate the model a several times for a range of parameter values (We just want you to explore a bit).




Problem 1C: Coding a very simple model.

Next, we will model an extremely simple system. Suppose we have a set of agents (called ‘turtles’ in NetLogo) which obey four simple rules at each time step:

The code for each rule is given below. Open the empty_example_model.nlogo provided, click on Code , and then modify it by adding the rule code provided below to the “go” function so that the turtles follow each of the four rules at each time step (check the comments in the empty_example_mode.nlogo for more details). Now run the model several times. What patterns do you see?

Note: if your NetLogo is not v6.0.1, there might be warning messages when you open the file. Please just hit Continue to go on.

Rule code:



Change the number of steps the turtles move forward by at each time step (start with a step size of 1, then try increasing it). How do the patterns change as you increase the step size?




Problem 2A: Compartmental models of contagion.

Next, we’ll develop and simulate a model of contagion. The model diagram and equations are:


\[\frac{dX_1}{dt} = -bX_1X_2+aX_2\] \[\frac{dX_2}{dt} = bX_1X_2-aX_2\]
We can use the same model to represent three different scenarios—choose one to use for this exercise:

  1. Infectious Disease Spread. In this scenario, \(X_1\) represents the number of susceptible individuals in the population, and \(X_2\) represents the number of infected individuals in the population. The term \(bX_1X_2\) represents the process by which susceptible become infected, and the \(aX_2\) term represents recovery of infected individuals (once recovered, individuals go back to the susceptible state).

  2. Tobacco Use. In this scenario, \(X_1\) represents the number of non-smoker individuals in the population, and \(X_2\) represents the number of smokers in the population. The term \(bX_1X_2\) represents the process by which non-smokers begin smoking, assuming that they begin smoking via peer pressure from smokers. The \(aX_2\) term represents smokers quitting smoking.

  3. Prion Conformational Changes. Prions are malformed proteins which can cause a variety of brain diseases. In this scenario, \(X_1\) represents the number of normal prion proteins in the tissue, and \(X_2\) represents the number of prions (malformed proteins) in the tissue. The term \(bX_1X_2\) represents the process by which normal proteins become malformed after interacting with a prion (malformed protein). We suppose that for this prion disease there is also a treatment which returns the proteins back to their normal state at rate \(aX_2\).

Implement and run the model in R, using either the R code below or the Shiny app (open model_shiny.Rmd, and click Smiley face), which uses an interface similar to NetLogo (i.e. you can use slider bars to change the value of parameters). What behavior do you observe? Is this what you would expect to happen for the scenario you chose?



R code :

library(deSolve)
## model equations
model1 <- function(t, x, params){
  x1 = x[1]
  x2 = x[2]
  
  b = params[1]
  a = params[2]
  
  dx1 = -b*x1*x2+a*x2
  dx2 = b*x1*x2-a*x2
  
  list(c(dx1, dx2))
}
## set up initial conditions and parameter values
ini_md1 <- c(x1=99, x2=1)
param_md1 <- c(b=0.01, a=0.1)

## time steps
times<- seq(0,20,by=1)

## model simulation
res_md1<- as.data.frame(ode(y=ini_md1, times=times, func=model1, parms=param_md1, method="ode45"))
#res_md1 #print out the results from each time step

To plot your results:

## plot epidemic curve
matplot(res_md1$time, as.data.frame(c(res_md1[2],res_md1[3])), type='l', xlab='time', ylab='counts', lwd=3, bty='l', col=c(1,3), cex.lab=1.3, cex.axis=1.1, ylim=c(0, 130))
legend("top",c("x1", "x2"), col=c(1,3),bty="n", horiz=TRUE, lwd=2, lty=1:3,cex=1.2 )


Problem 2B: Parameters and interpretation.

Using the model from Problem 2A, choose one parameter (a or b). Without running the model, predict how you would expect the model behavior to change if you increased that parameter by a factor of 5, and write down your prediction.

Now, change the parameter and simulate the model—does it match your prediction? Try this for the other parameter as well.



Problem 2C: Distinguishing alternative mechanisms.

Now, suppose we have two other candidate models which we might use: (R code is provided below, or you can use the same Shiny app file)

\[\frac{dX_1}{dt} = -bX_1X_2\]

\[\frac{dX_2}{dt} = bX_1X_2-rX_2\]

\[\frac{dX_3}{dt} =rX_2\]


\[\frac{dX_1}{dt} = -bX_1X_2+aX_3\]

\[\frac{dX_2}{dt} = bX_1X_2-rX_2\]

\[\frac{dX_3}{dt} =rX_2-aX_3\]


The alternate models above add to the model from Problem 2A, supposing that after \(X_2\) (infectious/smoker/prion), a person/molecule moves to a third state, \(X_3\).

  1. For Scenario 1, \(X_3\) represents the number of individuals who have immunity to the disease. In Alternate Model 2, they lose immunity and then return to being susceptible to the disease.

  2. For Scenario 2, \(X_3\) represents quit individuals who are resistant to smoking. We suppose that after quitting, an individual will have some resistance to re-starting smoking. In Alternate Model 2, after a while people in \(X_3\) lose their resistance and go back to being ordinary non-smokers (\(X_1\)).

  3. For Scenario 3, \(X_3\) represents prions which have been treated. In Alternate Model 2, after some time, they return to the normal protein state (\(X_1\)).



R Code for both models:

Alternate Model 1

## Alternate Model 1 - equations
model2 <- function(t, x, params){
  x1 = x[1]
  x2 = x[2]
  x3 = x[3]
  
  b = params[1]
  r = params[2]
  
  dx1 = -b*x1*x2
  dx2 = b*x1*x2-r*x2
  dx3 = r*x2
  
  list(c(dx1, dx2, dx3))
}
## set up initial conditions and parameter values
ini_md2 <- c(x1=99, x2=1, x3=0.0)
param_md2 <- c(b=0.01, r=0.02)

## time steps
times<- seq(0,20,by=1)

## model simulation
res_md2<- as.data.frame(ode(y=ini_md2, times=times, func=model2, parms=param_md2, method="ode45"))
#res_md2 #print out the results from each time step

To plot your results:

## plot epidemic curve
matplot(res_md2$time, as.data.frame(c(res_md2[2],res_md2[3],res_md2[4])), type='l', xlab='time', ylab='counts', lwd=3, bty='l', col=c(1,3,4), cex.lab=1.3, cex.axis=1.1, ylim=c(0, 130))
legend("top",c("x1", "x2", "x3"), col=c(1,3,4),bty="n", horiz=TRUE, lwd=2, lty=1:4,cex=1.2 )


Alternate Model 2

## Alternate Model 2 - equations
model3 <- function(t, x, params){
  x1 = x[1]
  x2 = x[2]
  x3 = x[3]
  
  b = params[1]
  a = params[2]
  r = params[3]
  
  dx1 = -b*x1*x2+a*x3
  dx2 = b*x1*x2-r*x2
  dx3 = r*x2-a*x3
  
  list(c(dx1, dx2, dx3))
}
## set up initial conditions and parameter values
ini_md3 <- c(x1=99, x2=1, x3=0.0)
param_md3 <- c(b=0.01, a=0.01, r=0.02)

## time steps
times<- seq(0,20,by=1)

## model simulation
res_md3<- as.data.frame(ode(y=ini_md3, times=times, func=model3, parms=param_md3, method="ode45"))
#res_md3 #print out the results from each time step

To plot your results:

## plot epidemic curve
matplot(res_md3$time, as.data.frame(c(res_md3[2],res_md3[3],res_md3[4])), type='l', xlab='time', ylab='counts', lwd=3, bty='l', col=c(1,3,4), cex.lab=1.3, cex.axis=1.1, ylim=c(0, 130))
legend("top",c("x1", "x2", "x3"), col=c(1,3,4),bty="n", horiz=TRUE, lwd=2, lty=1:4,cex=1.2 )