[R] segmented do not correctly fit data (variable names problem)

[Bert Gunter]

Note: My warning was for "stepwise" regression, which is what *you wrote*,
not "segmented".

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 Wed, Jun 10, 2020 at 3:09 AM PIKAL Petr <petr.pikal using precheza.cz> wrote:

> Dear all
>
> To make my problem more on topic I would like to ask about weird results
> from segmented fit, despite of Bert's warning.
>
> Here is my data
>
> temp <- structure(list(V1 = c(0L, 15L, 30L, 45L, 60L, 75L, 90L, 105L, 120L,
> 135L, 150L, 165L, 180L, 195L, 210L, 225L, 240L, 255L, 270L, 285L, 300L,
> 315L, 330L, 345L, 360L), V2 = c(98.68666667, 100.8, 103.28, 107.44, 110.06,
> 114.26, 117.6, 121.04, 123.8533333, 126.66, 129.98, 134.1866667, 139.04,
> 144.6, 152.08, 161.3, 169.8733333, 176.6133333, 181.92, 186.0266667,
> 188.7533333, 190.7066667, 192.0533333, 192.9933333, 193.3533333)), class =
> "data.frame", row.names = c(NA,
> + -25L))
>
> Here is the fit.
>
> library(segmented)
> plot(temp$V1, temp$V2)
> fit <- lm(V2~V1, temp)
> fit.s <- segmented(fit, seg.Z = ~ V1, npsi=2)
> plot(fit.s, add=TRUE, col=2)
>
> which is wrong.
>
> If I take example from web, the result is OK.
>
> set.seed(12)
> xx <- 1:100
> zz <- runif(100)
> yy <- 2 + 1.5*pmax(xx - 35, 0) - 1.5*pmax(xx - 70, 0) + 15*pmax(zz - .5, 0)
> +  rnorm(100,0,2)
> dati <- data.frame(x = xx, y = yy, z = zz)
> out.lm <- lm(y ~ x, data = dati)
> o <- segmented(out.lm, seg.Z = ~x, psi = list(x = c(30,60)),  control =
> seg.control(display = FALSE)
> )
> plot(dati$x, dati$y)
> plot(o, add=TRUE, col=2)
>
> What am I doing wrong? Is there a bug in segmented? BTW, if I change column
> names in temp to x and y, segmented found correct fit.
>
> names(temp) <- c("x", "y")
> plot(temp$x, temp$y)
> fit <- lm(y~x, temp)
> fit.s <- segmented(fit, seg.Z = ~x, npsi=2)
> plot(fit.s, add=TRUE, col=2)
>
> > sessionInfo()
> R Under development (unstable) (2020-03-08 r77917)
> Platform: x86_64-w64-mingw32/x64 (64-bit)
> Running under: Windows 10 x64 (build 18363)
>
> Matrix products: default
>
> locale:
> [1] LC_COLLATE=Czech_Czechia.1250  LC_CTYPE=Czech_Czechia.1250
> [3] LC_MONETARY=Czech_Czechia.1250 LC_NUMERIC=C
> [5] LC_TIME=Czech_Czechia.1250
>
> attached base packages:
> [1] stats     graphics  grDevices utils     datasets  methods   base
>
> other attached packages:
> [1] segmented_1.1-0
>
> loaded via a namespace (and not attached):
> [1] compiler_4.0.0 tools_4.0.0    splines_4.0.0
>
>
> Cheers
> Petr
>
>
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