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Ensemble Output Validation (VVP3) of Multiscale Migration Prediction Simulations

In this tutorial we showcase the ensemble output validation pattern, or VVP3. VVP3 is a pattern that allows users to execute multiple simulations, and perform a streamlined validation procedure across the results of all these simulations.

Our VVP3 implementation applies a sample testing function on each directory, and prints the outputs to screen. It then uses an aggregation function to combine all outputs into a compound metric.

As an example application of VVP3, we developed a validate_flee and a validate_flee_output function in FabFlee, which apply the VVP to sets of forced migration simulaitons. The VVP can be applied to one or more simulations, and supports the use of a single input configuration (or conflict situation in the context of these models), as well as a range of different input configurations.

Throughout the tutorial we use the fab command, but it is also possible to use the fabsim command if you have set your paths correctly. The latter command will give clearer error reporting when you mistype your instruction.

Ensemble Validation of a single conflict with multiple instances

In this first step, we will demonstrate the use of VVP3 with a forecast simulation of a conflict in Ethiopia. In the config_files/ethiopia directory we created an ensemble of configurations, where each configuration differs in the way that the conflict (hypothetically) progresses. All these configurations can be found in config_files/ethiopia/SWEEP.

  1. In general, to execute multiple instances of a single conflict scenario, you'll need to run the flee_ensemble function as follows:

    fab localhost flee_ensemble:<conflict_name>,simulation_period=<number>
    

    or

    fab <remote machine name> flee_ensemble:<conflict_name>,simulation_period=<number>
    

    NOTE : Any conflict scenario in the config_files run with flee_ensemble should contain a SWEEP directory with different configurations for each instance.

    For instance, to execute ensemble simulations for the Ethiopia conflict, simply type:

    fab localhost flee_ensemble:ethiopia,simulation_period=147
    

    or

    fab eagle_vecma flee_ensemble:ethiopia,simulation_period=147
    
  2. You can then copy back any results from completed runs using:

    fab localhost fetch_results
    

    To check the executed output directory, simply run:

    cd (FabSim3 Home)/results
    ls
    

    In (FabSim3 Home)/results, an output directory for a conflict scenario should be present from previous execution, which is most likely called <conflict name>_localhost_16 or <conflict name>_<remote machine name>_<number> (e.g. ethiopia_localhost_16 or ethiopia_eagle_vecma_4). This conflict output directory should have multiple run directories in RUNS.

  3. To run validation on these output directories of a single conflict with multiple instances, simply go to the FabFlee directory and run:

    fab localhost validate_flee_output:<conflict name>_localhost_16 
    

    or

    fab <remote machine name> validate_flee_output:<conflict name>_<remote machine name>_<number of cores used> 
    

    For instance, to validate the Ethiopia conflict instance, simply type:

    fab localhost validate_flee_output:ethiopia_localhost_16
    

    or

    fab eagle_vecma validate_flee_output:ethiopia_eagle_vecma_4 
    

    You should then see output similar to the picture below: "Example output"

    Here, the mean score indicates the averaged relative difference between the camp arrival numbers in the simulation versus those observed by UNHCR. The average is performed across all simulations in the ensemble. A value of 1.0 indicates that the forecast is 50% wrong, while a value of 0.0 indicates that the forecast is entirely correct.

Ensemble Validation of a single conflict instance with replicas

Like many other simulation codes out there, Flee is a non-deterministic code which means that results can vary from run to run. To help account for the uncertainty introduced by non-deterministic elements in the code, we can runs multiple identical copies (or replicas) of each individual simulation in our ensemble. Doing so is fairly straightforward, and you can do it as follows:

  1. To run simulations by specifying multiple replicas, you can use a function in this format:

    fab localhost flee:<conflict_name>,simulation_period=<number>,replicas=<number>
    

    or

    fab <remote machine name> flee:<conflict_name>,simulation_period=<number>,replicas=<number>
    

    For instance, to execute an ensemble simulations for the Mali conflict with two replicase, you can type:

    fab localhost flee:mali,simulation_period=300,replicas=2
    

    or

    fab eagle_vecma flee:mali,simulation_period=300,replicas=2
    
  2. Next, copy back any results from completed runs using:

    fab localhost fetch_results
    

    This will copy the output directory of a conflict instance with multiple replicas to (FabSim3 Home)/results, which are most likely called <conflict name>_localhost_16_replica_<number> or <conflict name>_<remote machine name>_<number>_replica_<number> (e.g. mali_localhost_16_replica_1 or mali_eagle_vecma_4_replica_2).

  3. To run an ensemble validation for these replica instances, make a directory with subdirectory <conflict_name>/RUNS in (FabSim3 Home)/results and place replica runs inside. This will provide average over all those replicas for each conflict instance.

    For instance, there are mali_localhost_16_replica_1 and mali_localhost_16_replica_2 output directories, which are placed inside mali/RUNS for ensemble validation.

  4. To run validation on these output directories of a single instance with replica instances, simply go to the FabFlee directory and run

    fab localhost validate_flee_output:<conflict name>
    

    or

    fab <remote machine name> validate_flee_output:<results_directory_name>
    

    For instance, to validate your Mali conflict instance run on localhost, simply type

    fab localhost validate_flee_output:validation_localhost_4
    

    or

    fab localhost validate_flee_output:validation_eagle_4
    
    

Ensemble Validation of multiple conflict instances

To execute multiple conflict scenarios, simply run the validate_flee function as follows

fab localhost validate_flee

or

fab <remote machine name> validate_flee

which executes multiple simulations in FabFlee/config_files/validation/SWEEP containing burundi2015, car2013 and other conflict instances.

After simulations are completed, validation values of these conflict instances are calculated and printed to the terminal screen.