# Understanding the Model Network¶

At its core, metawards implements a meta-population model. Individuals are grouped into workers and players, and distributed across wards. The force of infection of a ward caused by infection of an individual, based on their movements and behaviour is calculated, and used to decide whether other individuals who reside or visit that ward should be infected.

The Network is the collection of Nodes and Links that describe the individual wards, and the movements between wards of their residents.

Up to now, you have used a single underlying Network for every demographic and model run in this tutorial. This does not need to be the case, and you can choose to assign different Network objects to different demographics in a simulation. You would do this, for example, to model different connections or movements between wards for different demographics, e.g. workers or school students. Or to use different networks to represent movements related to holidays or foreign travel.

## The single-node network¶

You can set the overall network used by default by all demographics via the --model or -m command-line argument. The default value is 2011Data, which is based on 2011 census data, and models every electoral ward in England and Wales.

There are two other models supplied;

• 2011UK : Again, this is based on the 2011 census, but includes Scotland and Northern Ireland. This model is still a work in progress.

• single : This is a single-ward network that is used for testing, or when you don’t want to model geographic behaviour.

You can run a single-ward simulation using the command line;

metawards -d lurgy_home -m single -P 100


Note

We are using the lurgy_home.json file from the last chapter of this tutorial. Note that we also need to set the population of this single ward using the -P or --population command line argument. In this case, we are setting the population to 100 individuals.

When you run, you should see that metawards is siginficantly faster. Also, as the outbreak is not seeded, nothing much happens.

## Seeding the outbreak¶

Up to now, you have been using the ExtraSeedsLondon.dat file to seed every outbreak. This file contains the single line;

1       5   255


The three numbers instruct metawards to seed the outbreak on day 1 (the first number), infecting 5 individuals (the second number) in ward 255 (the third number).

Warning

The name of this file is misleading, as it is really seeding the ward with index 255, not seeding a ward in London. This will only be seeding London if the network this is used with has a ward in London at index 255.

If you try to seed using this file you will get an error, e.g.

metawards -d lurgy_home -m single -P 100 -a ExtraSeedsLondon.dat

━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ ERROR ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
Unable to seed the infection using (1, 255, 5, None). The error was <class 'IndexError'>: Invalid node index
255. Number of nodes in this container equals 2. Please double-check that you are trying to seed a node that
exists in this network.

━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
Traceback (most recent call last):

[lots of output]

IndexError: Invalid node index 255. Number of nodes in this container equals 2


Note

The traceback can be long and complex, and is really only of use for metawards developers. You can normally work out what has gone wrong by scrolling up to before the traceback, and seeing if there is a ERROR printed immediately before.

We have provided an ExtraSeedsOne.dat file, which seeds 5 infections in ward 1 on the first day of the outbreak. This contains the line;

1       5        1


which says to seed 5 individuals on the first day in ward 1.

Note

Nodes are indexed from 1 rather than 0. This means that ward[1] is the first node, and ward[nnodes] is the last node in the network.

You can use this with single-ward networks, e.g.

metawards -d lurgy_home -m single -P 100 -a ExtraSeedsOne.dat


However, creating a file to seed an outbreak is inconvenient, particularly when you only want to seed a single ward. You can, optionally, pass the seeding information as the argument, instead of the filename. Thus this will work;

metawards -d lurgy_home -m single -P 100 -a "1 5 1"


Equally, you can seed on multiple days, e.g. seeding 5 individuals on day 1, and then 10 individuals on day 2, via;

metawards -d lurgy_home -m single -P 100 -a "1 5 1\n2 10 1"


The contents of the string is interpreted identically to if it had been read from a file, with \n representing a newline character.

Note

The extra seeds file has a flexible and powerful format, e.g. supporting seeding by date, seeding random wards or by random amounts etc. More information on the format of this file can be found here.