[R] How important is set.seed

Bert Gunter
Tue, 22 Mar 2022 16:12:00 -0000

OK, I'm somewhat puzzled by this discussion. Maybe I'm just clueless.
But...

1. set.seed() is used to make any procedure that uses R's
pseudo-random number generator -- including, for example, sampling
from a distribution, random data splitting, etc. -- "reproducible".
That is, if the procedure is repeated *exactly,* by invoking
set.seed() with its original argument values (once!) *before* the
procedure begins, exactly the same results should be produced by the
procedure. Full stop. It does not matter how many times random number
generation occurs within the procedure thereafter -- R preserves the
state of the rng between invocations (but see the notes in ?set.seed
for subtle qualifications of this claim).

2. Hence, if no (pseudo-) random number generation is used, set.seed()
is irrelevant. Full stop.

3. Hence, if you don't care about reproducibility (you should! -- if
for no other reason than debugging), you don't need set.seed()

4. The "randomness" of any sequence of results from any particular
set.seed() arguments (including further calls to the rng) is a complex
issue. ?set.seed has some discussion of this, but one needs
considerable expertise to make informed choices here. As usual, we
untutored users should be guided by the expert recommendations of the
Help file.

*** If anything I have said above is wrong, I would greatly appreciate
a public response here showing my error.***

Bert Gunter

"The trouble with having an open mind is that people keep coming along
and sticking things into it."
-- Opus (aka Berkeley Breathed in his "Bloom County" comic strip )



On Tue, Mar 22, 2022 at 7:48 AM Neha gupta <neha.bologna90 at gmail.com>
wrote:
>
> Hello Tim
>
> In some of the examples I see in the tutorials, they put the random seed
> just before the model training e.g train function in case of caret library.
> Should I follow this?
>
> Best regards
> On Tuesday, March 22, 2022, Ebert,Timothy Aaron <tebert at ufl.edu>
wrote:
>
> > Ah, so maybe what you need is to think of ?set.seed()? as a treatment
in
> > an experiment. You could use a random number generator to select an
> > appropriate number of seeds, then use those seeds repeatedly in the
> > different models to see how seed selection influences outcomes. I am
not
> > quite sure how many seeds would constitute a good sample. For me that
would
> > depend on what I find and how long a run takes.
> >
> >   In parallel processing you set seed in master and then use a random
> > number generator to set seeds in each worker.
> >
> > Tim
> >
> >
> >
> > *From:* Neha gupta <neha.bologna90 at gmail.com>
> > *Sent:* Tuesday, March 22, 2022 6:33 AM
> > *To:* Ebert,Timothy Aaron <tebert at ufl.edu>
> > *Cc:* Jeff Newmiller <jdnewmil at dcn.davis.ca.us>; r-help at
r-project.org
> > *Subject:* Re: How important is set.seed
> >
> >
> >
> > *[External Email]*
> >
> > Thank you all.
> >
> >
> >
> > Actually I need set.seed because I have to evaluate the consistency of
> > features selection generated by different models, so I think for this,
it's
> > recommended to use the seed.
> >
> >
> >
> > Warm regards
> >
> > On Tuesday, March 22, 2022, Ebert,Timothy Aaron <tebert at
ufl.edu> wrote:
> >
> > If you are using the program for data analysis then set.seed() is not
> > necessary unless you are developing a reproducible example. In a
standard
> > analysis it is mostly counter-productive because one should then ask
if
> > your presented results are an artifact of a specific seed that you
selected
> > to get a particular result. However, in cases where you need a
reproducible
> > example, debugging a program, or specific other cases where you might
need
> > the same result with every run of the program then set.seed() is an
> > essential tool.
> > Tim
> >
> > -----Original Message-----
> > From: R-help <r-help-bounces at r-project.org> On Behalf Of Jeff
Newmiller
> > Sent: Monday, March 21, 2022 8:41 PM
> > To: r-help at r-project.org; Neha gupta <neha.bologna90 at
gmail.com>; r-help
> > mailing list <r-help at r-project.org>
> > Subject: Re: [R] How important is set.seed
> >
> > [External Email]
> >
> > First off, "ML models" do not all use random numbers (for
prediction I
> > would guess very few of them do). Learn and pay attention to what the
> > functions you are using do.
> >
> > Second, if you use random numbers properly and understand the
precision
> > that your specific use case offers, then you don't need to use
set.seed.
> > However, in practice, using set.seed can allow you to temporarily
avoid
> > chasing precision gremlins, or set up specific test cases for testing
code,
> > not results. It is your responsibility to not let this become a
crutch... a
> > randomized simulation that is actually sensitive to the seed is
unlikely to
> > offer an accurate result.
> >
> > Where to put set.seed depends a lot on how you are performing your
> > simulations. In general each process should set it once uniquely at
the
> > beginning, and if you use parallel processing then use the features of
your
> > parallel processing framework to insure that this happens. Beware of
> > setting all worker processes to use the same seed.
> >
> > On March 21, 2022 5:03:30 PM PDT, Neha gupta <neha.bologna90 at
gmail.com>
> > wrote:
> > >Hello everyone
> > >
> > >I want to know
> > >
> > >(1) In which cases, we need to use set.seed while building ML
models?
> > >
> > >(2) Which is the exact location we need to put the set.seed
function i.e.
> > >when we split data into train/test sets, or just before we train a
model?
> > >
> > >Thank you
> > >
> > >       [[alternative HTML version deleted]]
> > >
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>         [[alternative HTML version deleted]]
>
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