[R] Question about "sample" function and inconsistent results I am getting across machines.

[Fomby, Tom]

Dear Duncan,

OK, I will certainly ask my students to download the most recent version of Basic R at the first of each semester and, just to be safe, include the RNGkind(sample.kind="Rejection") command before the students get started on the data partitioning part of their exercise using the sample function.

By the way, how is it that one can take a membership in the R community so as to provide support for volunteers like yourself.

Thank you,

Tom Fomby

Department of Economics

SMU

Dallas, TX 75275


________________________________
From: Duncan Murdoch <murdoch.duncan using gmail.com>
Sent: Sunday, May 3, 2020 2:32 PM
To: Fomby, Tom; r-help using R-project.org
Subject: Re: [R] Question about "sample" function and inconsistent results I am getting across machines.

On 03/05/2020 1:39 a.m., Fomby, Tom wrote:
> Please consider the following code:
>
> set.seed(1)
>
> train.index = sample(181,150)
> head(train.index)
> # [1]  49  67 103 162  36 159  Result from my ASUS computer
> #
> # [1]  68 167 129 162 43 14  Result from my wife's HP Pavilion computer
>
> In both cases, version 3.6.3 of R are being used.
>
> In addition, of the 20 students in my Predictive Analytics class, 14 got the first result while 6 got the latter result.  These results do not seem to be specific to MAC (OS) versus PC (Windows).  In several cases, students using 3.6.3 got differing results.  This makes grading of homework challenging not knowing which partitions of the data are being used by the student.
>
> Thank you for considering my question.

Likely some of you are storing and restoring workspaces, and have been
doing so for a long time.  If you type

RNGkind()

what you should see is

[1] "Mersenne-Twister" "Inversion"        "Rejection"

but if the .Random.seed is restored from an old session, you might see

[1] "Mersenne-Twister" "Inversion"        "Rounding"

The latter uses the buggy version of sample().  Those users should run

RNGkind(sample.kind = "Rejection")

to start using the corrected sampling algorithm.  (The default was
changed in R 3.6.0, but if you saved your seed from a previous version,
you'd get the old sampler).

They should also stop reloading old workspaces, but that's another
discussion.

Duncan Murdoch

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