[R] cut{base}: is it a bug?
David L Carlson
dc@r|@on @end|ng |rom t@mu@edu
Mon Sep 24 19:42:52 CEST 2018
Yes, I should have included that point. The cut() function "encourages" exact comparison of values by including the right= argument without a warning that this may create unexpected results. With truly continuous data, values falling exactly on the boundary would be rare.
Most data arrives from instruments that measure to limited precision. Introductory statistics texts deal with this by distinguishing between "true" and "stated" class limits. Or, like Lyman Ott, recommend choosing the starting point interval such that "no measurement falls on a point of division between two intervals."
----------------------------------------
David L Carlson
Department of Anthropology
Texas A&M University
College Station, TX 77843-4352
-----Original Message-----
From: Jeff Newmiller <jdnewmil using dcn.davis.ca.us>
Sent: Monday, September 24, 2018 10:41 AM
To: r-help using r-project.org; David L Carlson <dcarlson using tamu.edu>; Jose Claudio Faria <joseclaudio.faria using gmail.com>; r-help using r-project.org
Subject: Re: [R] cut{base}: is it a bug?
"Subtracting a bit" only fixes the problem for the test data... it introduces a bias in any continuous data you happen to throw at it. However, if you have data with known rounding applied (e.g. published tabular data) then the subtracting trick can be useful. In general you should not expect floating point fractions to behave like exact values in your analysis.
On September 24, 2018 8:14:09 AM PDT, David L Carlson <dcarlson using tamu.edu> wrote:
>You've been bitten by FAQ 7.31: Why doesn't R think these numbers are
>equal?
>https://cran.r-project.org/doc/FAQ/R-FAQ.html#Why-doesn_0027t-R-think-these-numbers-are-equal_003f
>
>Your boundaries and your data values are not what you think they are.
>This is a limitation of digital computing not R.
>
>> print(seq(from=.6, to=2.2, by=.2), digits=17)
>[1] 0.59999999999999998 0.80000000000000004 1.00000000000000000
>1.20000000000000018
>[5] 1.39999999999999991 1.60000000000000009 1.80000000000000027
>2.00000000000000000
>[9] 2.20000000000000018
>
>> print(dat, digits=17)
>[1] 0.59999999999999998 0.59999999999999998 0.59999999999999998
>0.69999999999999996
>[5] 0.69999999999999996 0.69999999999999996 0.69999999999999996
>0.69999999999999996
>[9] 0.80000000000000004 0.80000000000000004 0.80000000000000004
>0.90000000000000002
>[13] 0.90000000000000002 0.90000000000000002 0.90000000000000002
>1.00000000000000000
>[17] 1.00000000000000000 1.00000000000000000 1.00000000000000000
>1.10000000000000009
>[21] 1.10000000000000009 1.10000000000000009 1.19999999999999996
>1.19999999999999996
>[25] 1.19999999999999996 1.19999999999999996 1.30000000000000004
>1.30000000000000004
>[29] 1.30000000000000004 1.39999999999999991 1.39999999999999991
>1.39999999999999991
>[33] 1.50000000000000000 1.50000000000000000 1.50000000000000000
>1.60000000000000009
>[37] 1.60000000000000009 1.69999999999999996 1.69999999999999996
>1.69999999999999996
>[41] 1.69999999999999996 1.80000000000000004 1.80000000000000004
>1.80000000000000004
>[45] 1.89999999999999991 1.89999999999999991 2.00000000000000000
>2.00000000000000000
>[49] 2.00000000000000000 2.00000000000000000 2.00000000000000000
>2.10000000000000009
>
>The simplest solution is to subtract a bit. This also means you don't
>need the include.lowest= or right= arguments:
>
>> f <- cut(dat,
>+ breaks= seq(from=.6-.01, to=2.2-.01, by=.2),
>+ dig.lab=10L)
>> as.matrix(tb <- table(f))
> [,1]
>[0.59,0.79) 8
>[0.79,0.99) 7
>[0.99,1.19) 7
>[1.19,1.39) 7
>[1.39,1.59) 6
>[1.59,1.79) 6
>[1.79,1.99) 5
>[1.99,2.19] 6
>
>----------------------------------------
>David L Carlson
>Department of Anthropology
>Texas A&M University
>College Station, TX 77843-4352
>
>
>-----Original Message-----
>From: R-help <r-help-bounces using r-project.org> On Behalf Of Jose Claudio
>Faria
>Sent: Monday, September 24, 2018 9:32 AM
>To: r-help using r-project.org
>Subject: [R] cut{base}: is it a bug?
>
>Dears members,
>
>Is the below a bug of the cut {base} function?
>
>dat <- c(
> 0.6, 0.6, 0.6, 0.7, 0.7, 0.7, 0.7, 0.7, #(8)
> 0.8, 0.8, 0.8, 0.9, 0.9, 0.9, 0.9, #(7)
> 1.0, 1.0, 1.0, 1.0, 1.1, 1.1, 1.1, #(7)
> 1.2, 1.2, 1.2, 1.2, 1.3, 1.3, 1.3, #(7)
> 1.4, 1.4, 1.4, 1.5, 1.5, 1.5, #(6)
> 1.6, 1.6, 1.7, 1.7, 1.7, 1.7, #(6)
> 1.8, 1.8, 1.8, 1.9, 1.9, #(5)
> 2.0, 2.0, 2.0, 2.0, 2.0, 2.1 #(6)
> )
>
># making class from function "cut"
>(f <- cut(dat,
> breaks= seq(from=.6, to=2.2, by=.2),
> include.lowest=TRUE,
> dig.lab=10L,
> right=FALSE))
>
># more easy to see the table
>as.matrix(tb <- table(f))
>
># Checking
>print(length(dat[dat >= 0.6 & dat < 0.8])) == tb[1]
>print(length(dat[dat >= 0.8 & dat < 1.0])) == tb[2]
>print(length(dat[dat >= 1.0 & dat < 1.2])) == tb[3] # !?
>print(length(dat[dat >= 1.2 & dat < 1.4])) == tb[4] # !?
>print(length(dat[dat >= 1.4 & dat < 1.6])) == tb[5]
>print(length(dat[dat >= 1.6 & dat < 1.8])) == tb[6] # !?
>print(length(dat[dat >= 1.8 & dat < 2.0])) == tb[7] # !?
>print(length(dat[dat >= 2.0 & dat < 2.2])) == tb[8]
>
>Best,
>///\\\///\\\///\\\///\\\///\\\///\\\///\\\///\\\
>Jose Claudio Faria
>UESC/DCET/Brasil
>joseclaudio.faria at gmail.com
>Telefones:
>55(73)3680.5545 - UESC
>55(73)99966.9100 - VIVO
>55(73)98817.6159 - OI
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>
>If you have software to deal with statistics, you have arms; if you
>have good software, you have arms and legs; if you have software like
>R, you have arms, legs and wings...
>the height of your flight depends only on you!
>
> [[alternative HTML version deleted]]
>
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>
>______________________________________________
>R-help using r-project.org mailing list -- To UNSUBSCRIBE and more, see
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>PLEASE do read the posting guide
>http://www.R-project.org/posting-guide.html
>and provide commented, minimal, self-contained, reproducible code.
--
Sent from my phone. Please excuse my brevity.
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