Re: Not to incite a language war but apparently the Corona lockdown was based on 13 year old undocumented C-Code

[Niklas Holsti <niklas.holsti@tidorum.invalid>]

On 2020-05-13 1:39, Jeffrey R. Carter wrote:
> On 5/13/20 12:20 AM, Niklas Holsti wrote:
>>
>> I agree that it would be troubling. If you could find that statement, 
>> it would interest me.
> 
> In https://lockdownsceptics.org/code-review-of-fergusons-model/ it says, 
> "the code produces critically different results, even for identical 
> starting seeds and parameters."

While the review claims that as a general flaw, it then "illustrates" 
the claim by discussing the two cases I detailed, in particular the 
unexpected influence of the option controlling how the program "stores 
data tables". This was eventually traced to a difference in the way the 
program used the RNG, depending on this option, which then led to 
different PRN sequences, depending on this option. An error in the 
program, of course, but not non-determinism.

For programs that use RNGs to drive simulations, a single extra RNG 
call, or a single omitted RNG call, will completely change the 
subsequent PRN sequence. This has no effect on the statistical 
properties of the results from many runs, but will of course change the 
results of the particular run in which the change occurs.

> Initially it was claimed that this was 
> only true for multiple cores, but later that was retracted: "But 
> Edinburgh came back and reported that – even in single-threaded mode – 
> they still see the problem."

It seems that some users (Edinburgh) reported that the results varied, 
and reported they were using multi-core mode. The authors of the program 
(Imperial) replied that result variations are expected in multi-core 
mode; this was of course a sloppy analysis of the problem, but not an 
unnatural one. When the users reported that the problem occurs also in 
single-core mode, depending on the "data table storage" option, the 
authors found and fixed the error.

-- 
Niklas Holsti
niklas holsti tidorum fi
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