2. Getting started with EMULSION¶

We assume in what follows that you have downloaded and unzipped the models directory provided on EMULSION website.

2.1. Running EMULSION¶

To simulate an EMULSION model (named e.g. MODEL), open a terminal in the directory where your files are located, and type the following command:

emulsion run --plot MODEL.yaml


Option --plot automatically opens your web browser to show simulation outcomes. To view diagrams representing model structure, append option --view-model to the previous line.

As an example, open a terminal in the directory models/features and type:

emulsion run --plot compart_SIR.yaml --view-model


You can also check the results of a more complex model, e.g. quickstart, tested at install.

When running a model, EMULSION:

1. Reads file MODEL.yaml which describes the model
2. With options --view-model, builds the diagrams representing model structure and stores them in the figure directory (default: img/)
3. Runs N (default: 10) stochastic repetitions of model simulation during T (default: 100) time steps
4. Stores simulation outputs in a CSV file counts.csv located in the output directory (default: outputs/)
5. With option --plot, produces a web page with a basic representation of the outputs (and model structure if --view-model was also set) which is stored in the figure directory, under the name My_model_name.html (My_model_name being the name of the model as defined in MODEL.yaml by the instruction: model_name: My_model_name)

Most frequently used options are:

-r N
produce N stochastic repetitions
-t T
run the simulation for T time steps
--silent
show a progress bar only for the progression of repetitions (instead of a progress bar per repetition).
--output-dir OUTPUT
change output directory (the location of counts.csv produced by the simulation) to OUTPUT
--figure-dir FIG
change figure directory (the location of model diagrams and outputs web page) to FIG

(Other options are available with emulsion -h.)

2.2. Producing model diagrams¶

When running a model, you can use options --plot and --view-model to plot simulation outcomes and model diagrams, respectively.

To generate model diagrams without having to run the model, just type for instance:

emulsion diagrams compart_SIR.yaml


By default, diagrams are generated in SVG format (in the figure directory, by default img). You can change format to PNG or PDF with option --format, e.g.:

emulsion diagrams compart_SIR.yaml --format pdf


2.3. Viewing parameters¶

EMULSION command can also show the actual values of model parameters:

emulsion show compart_SIR.yaml


in the following list (“MODEL parameters”):

                      EmulsionModel "compart_SIR"
AVAILABLE PARAMETERS (with their current value)
------------------------------------------------------------------------
MODEL PARAMETERS
------------------------------------------------------------------------
total_duration.................................................100.0
initial_herd_size..............................................100.0
initial_infected.................................................1.0
transmission_I...................................................0.5
recovery.........................................................0.1
delta_t..........................................................1.0
------------------------------------------------------------------------


You can also ask for the role of the parameters:

emulsion describe compart_SIR.yaml recovery delta_t transmission_I


results in the following list (“MODEL parameters”):

                      EmulsionModel "compart_SIR"
ROLE OF PARAMETERS (AND CURRENT DEFINITION)
------------------------------------------------------------------------
recovery [parameter]:
recovery rate (/day)
0.1
delta_t [parameter]:
duration of the simulation time step
1.0
transmission_I [parameter]:
transmission rate from infectious animals (/day)
0.5
------------------------------------------------------------------------


2.4. Changing parameters¶

Now, to run the model with other parameter values without changing the model file, you can specify the new values with option -p PARAM=VALUE (one for each parameter to change), for instance:

emulsion run --plot compart_SIR.yaml -p transmission_I=0.3 -p recovery=0.01


which produces:

2.5. Changing the model¶

To change model structure and assumptions, open the file in a text editor. For instance, to change the above SIR model into a SEIRS model, open compart_SIR.yaml and save it as a new file, e.g. compart_SEIRS.yaml. You have to add a state (E), modify existing transitions and add new ones, and introduce new parameters (to represent the incubation and the waning of immunity).

1. Add a latent (or exposed) state: search for the health_state state machines and change the states and transitions as follows:

   state_machines:
health_state:
desc: 'The state machine which defines the evolution of health states'
states:
- S:
name: 'Susceptible'
desc: 'suceptible of becoming infected'
fillcolor: 'deepskyblue'
- E:
name: 'Latent'
desc: 'infected but not yet able to transmit the disease'
fillcolor: 'orange'
- I:
name: 'Infectious'
desc: 'infected and able to transmit the disease'
fillcolor: 'red'
- R:
name: 'Resistant'
desc: 'healthy again and resistant to infection'
fillcolor: 'limegreen'
transitions:
- {from: S, to: E, rate: 'force_of_infection'}
- {from: E, to: I, rate: '1/incubation'}
- {from: I, to: R, rate: 'recovery'}

2. Add the waning of immunity simply by adding the following transition:

- {from: R, to: S, rate: 'waning'}

3. Add the new parameters to the parameters section:

parameters:
...
incubation:
desc: 'mean duration of latent state (days, assuming exponential
distribution of durations in the state)
'
value: 5
waning:
desc: 'rate at which individuals lose immunity (/day)'
value: 0.05

4. Do not forget to update the calculation of prevalence, which is an expression also defined in the parameters section:

parameters:
...
'prevalence (%)':
desc: 'proportion of infected+infectious individuals'
value: '100 * (total_I + total_E) / total_population'


You’re done ! Test your model with emulsion command as in previous examples:

emulsion run --plot compart_SEIRS.yaml --view-model --silent


which produces also a new diagram:

2.6. Going further…¶

Check our examples

EMULSION provides a large number of modelling features. To go further on using EMULSION to design models according to your needs (even complex), jump to next sections: