Adjustable Variables¶
We have now adjusted the disease file for the lurgy, which has resulted in a model run that has involved a larger section of the population in the disease outbreak. However, the values we chose for the parameters in the disease file were made up. They had no basis in reality, which means that it is difficult to use this model to make good predictions.
metawards is designed to support parameter searches that seek to
adjust the disease parameters so that predictions from model runs
can be matched to observed reality. The idea is that variables are
adjusted until the model runs can reliably capture the observed
behaviour of an outbreak, at which point the model may then act
as a good prediction of the future.
Choosing what to adjust¶
In the last section we introduced a new stage to the lurgy in which infected individuals were very mobile (too_ill_to_move was 0.0), but also very infectious (beta was 0.5).
This was introduced as stage 3 of the lurgy. We need therefore to tune the beta[2] and too_ill_to_move[2] parameters (remembering that we count from zero). What we want to do is vary these two parameters and see how they affect the model outbreak.
To do this, in your current directory create a file called lurgyparams.csv
and copy in the text below;
beta[2] too_ill_to_move[2]
0.3 0.00
0.4 0.00
0.5 0.00
0.3 0.25
0.4 0.25
0.5 0.25
0.3 0.50
0.4 0.50
0.5 0.50
This file has two columns of numbers; beta[2] which shows how to
adjust the beta[2] value, and too_ill_to_move[2] which shows
how to adjust the too_ill_to_move[2] value. Each row of this file
represents a single model run using this pair of values. For example,
the line 0.3 0.00 will set beta[2] to 0.3 and
too_ill_to_move[2] to 0.0.
Note
The format of this file is very flexible. Columns can be space-separated
or comma-separated, as long as this is consistent in the entire file.
You can choose to adjust any of the parameters for any stage
of a disease. Simply title the column with the parameter name, e.g.
beta, progress, too_ill_to_move or contrib_foi, and
add the stage number in square brackets, e.g. beta[0]. Remember
that we count from zero, so stage one has index zero.
You can pass metawards this input file using the --input
(or -i) command-line option. This input file has nine pairs
of values of beta[2] and too_ill_to_move[2], which means that
nine model runs need to be performed. To run them all, use this command;
metawards -d lurgy2 -a ExtraSeedsLondon.dat --input lurgyparams.csv
This will run all nine jobs. If you have nine or more processor cores on your computer then all of them will be run in parallel (with individual model runs then using any other cores you have). If, like me, you are running this on your laptop, then this may take 10+ minutes to complete.
Note
If you want to distribute this work over a set of disconnected
computers, you can tell metawards to only adjust parameters
from a subset of the lines of the input file. To do this,
use the --line (or -l) command line argument to specify
the line or lines to process. Lines are counted from 0 being the
top line (containing the header), and multiple lines or ranges
can be specified, e.g. -l 1 2 3 will use lines one to three,
while -l 4-6 would use lines four to six (inclusive).
Warning
We are only going to use one repeat of each pair of values, which means
that our results will suffer from a lot of random error. Ideally you
should ask for multiple repeats using the --repeats command-line
argument. A good value would be at least eight repeats. In this case,
eight repeats would require 72 model runs. If you parallelise
each run over 16 cores then this needs 1152 cores. Fortunately, for
these jobs, metawards runs well in parallel
across a slurm or PBS cluster.
Once the jobs are complete (which took 14 minutes on my laptop), you should have output that looks similar to this;
Running 9 jobs using 4 process(es)
Running jobs in parallel using a multiprocessing pool...
Completed job 1 of 9
(beta[2]=0.3, too_ill_to_move[2]=0.0)[repeat 1]
2020-11-10: DAY: 204 S: 7976329 E: 0 I: 0 R: 48105748 IW: 0 TOTAL POPULATION 56082077
Completed job 2 of 9
(beta[2]=0.4, too_ill_to_move[2]=0.0)[repeat 1]
2020-10-29: DAY: 192 S: 7039360 E: 0 I: 0 R: 49042717 IW: 0 TOTAL POPULATION 56082077
Completed job 3 of 9
(beta[2]=0.5, too_ill_to_move[2]=0.0)[repeat 1]
2020-10-12: DAY: 175 S: 6224108 E: 0 I: 0 R: 49857969 IW: 0 TOTAL POPULATION 56082077
Completed job 4 of 9
(beta[2]=0.3, too_ill_to_move[2]=0.25)[repeat 1]
2020-11-11: DAY: 205 S: 8005734 E: 0 I: 0 R: 48076343 IW: 0 TOTAL POPULATION 56082077
Completed job 5 of 9
(beta[2]=0.4, too_ill_to_move[2]=0.25)[repeat 1]
2020-11-16: DAY: 210 S: 7073043 E: 0 I: 0 R: 49009034 IW: 0 TOTAL POPULATION 56082077
Completed job 6 of 9
(beta[2]=0.5, too_ill_to_move[2]=0.25)[repeat 1]
2020-10-20: DAY: 183 S: 6260111 E: 0 I: 0 R: 49821966 IW: 1 TOTAL POPULATION 56082077
Completed job 7 of 9
(beta[2]=0.3, too_ill_to_move[2]=0.5)[repeat 1]
2020-11-21: DAY: 215 S: 8045175 E: 0 I: 0 R: 48036902 IW: 0 TOTAL POPULATION 56082077
Completed job 8 of 9
(beta[2]=0.4, too_ill_to_move[2]=0.5)[repeat 1]
2020-11-06: DAY: 200 S: 7106554 E: 0 I: 0 R: 48975523 IW: 1 TOTAL POPULATION 56082077
Completed job 9 of 9
(beta[2]=0.5, too_ill_to_move[2]=0.5)[repeat 1]
2020-10-13: DAY: 176 S: 6304109 E: 0 I: 0 R: 49777968 IW: 0 TOTAL POPULATION 56082077
Writing a summary of all results into the
csv file /Users/chris/GitHub/tutorial/output/results.csv.bz2.