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 <jdnew...@dcn.davis.ca.us>
Sent: Monday, September 24, 2018 10:41 AM
To: r-help@r-project.org; David L Carlson <dcarl...@tamu.edu>; Jose Claudio
Faria <joseclaudio.fa...@gmail.com>; r-help@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 <dcarl...@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-boun...@r-project.org> On Behalf Of Jose Claudio
>Faria
>Sent: Monday, September 24, 2018 9:32 AM
>To: r-help@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
>///\\\///\\\///\\\///\\\///\\\///\\\///\\\///\\\
>
>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]]
>
>______________________________________________
>R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see
>https://stat.ethz.ch/mailman/listinfo/r-help
>PLEASE do read the posting guide
>http://www.R-project.org/posting-guide.html
>and provide commented, minimal, self-contained, reproducible code.
>
>______________________________________________
>R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see
>https://stat.ethz.ch/mailman/listinfo/r-help
>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.
______________________________________________
R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
