[jfnixon]

If you read the lockdownskeptics cite, "hard to debug" is not the problem. Non-determinism in the output is the issue, and if this is indeed the case, why would anyone trust the results? Do a bunch of runs and average is not a good answer.

[creato]

It's really not enough to say. "Do a bunch of runs and average" is exactly how quite a bit of simulation software works. In this case, a small number of random outcomes early in the "pandemic" will have a large impact on the outcome.

Of course, this kind of uncertainty needs to be dealt with, and that may have been done by running the simulation code we are presented with multiple times. It may be necessary to read both the code and the associated papers to judge this correctly.

[thu2111]

The non-determinism that page talks about is coming from bugs like memory corruptions, floating point inaccuracies, initialisation order bugs. It's not an intentional part of the model. Averaging corrupted data is meaningless, it doesn't magically fix the corruption.

I really wonder what it would take for some people to lose faith in epidemiology. Has this field ever predicted an epidemic correctly? Is there any level of bugginess that would yield the output of these teams unacceptable, to them?

[creato]

> The non-determinism that page talks about is coming from bugs like memory corruptions, ..., initialisation order bugs.

Source? I haven't seen these specifically cited anywhere.

> floating point inaccuracies,

Combined with (safe) race conditions, this will cause non-determinism that would probably be considered OK.

In general: John Carmack looked at the code and thought it was OK for what it is (decade old simulation transpiled from Fortran at some point). Some ex-Google guy thinks its horrible.

I looked at the code myself briefly. I haven't formed a strong opinion about the code myself beyond "it's ugly and I don't want to work with it, glad it's not my problem." I am however objecting to some of the comments here that make it sound like it is obviously broken for reasons that they just don't understand.

I think a comment on the GitHub is relevant: https://github.com/mrc-ide/covid-sim/issues/175#issuecomment...

> To add to this, please read report 9 properly. The 500k UK prediction was if governments did nothing whatsoever - we never believed governments would do nothing but we modelled it as a base case, because that's part of what you do when you model.

> With the full social distancing the report suggestd it might be possible to reduce deaths perhaps to 20k - a death count we have already exceeded. Nobody here is laughing about that. The report was also very frank about the uncertainty involved in trying to predict what might happen at that stage

[SiempreViernes]

Exactly, the individual runs aren't actually the "deliverable" of the code; rather it is the average of many runs that represent the real result of the code.

Nothing presented clearly compromised the (supposed) reliability of the distributions produced, so the impact of these bugs beyond the inconvenience they add is unclear.

To be clear, it is certainly not true that removing these bugs will somehow prove that the model and its inputs themselves are correct.