Hello,
Tip: in a formula, pH^2 = pH*pH is an interaction.
pH^2 = pH*pH = pH + pH + pH:pH = pH
Try I(pH^2)
Hope this helps,
Rui Barradas
Às 12:07 de 02/04/20, Troels Ring escreveu:
Dear friends - I'm on Win10 with R 6.3.1 and have a very simple problem with
nls which apparently gives a decent fit to the parable below, even without
starting values. But when I then think I know the meaning of the three
parameters a, b, and d it goes very wrong. I guess I am again overlooking
something easy but cannot spot it.
BW
Troels Ring,
Aalborg, Denmark
aedf <- structure(list(Flux = c(-0.141256, -0.154709, -0.215247, -0.302691,
-0.32287, -0.511211, -0.605381, -0.813901, -1.11659, -1.76906
), pH = c(7.06273, 7.11182, 7.16182, 7.18818, 7.21455, 7.23818,
7.26273, 7.28455, 7.31182, 7.34364)), class = "data.frame",
row.names = c(NA,
-10L))
m <- with(aedf,nls(Flux~a + b*pH + d*pH^2))
with(aedf,plot(pH,Flux))
with(aedf,lines(pH,fitted(m),lty=1,col="red",lwd=3))
m
# a b d
# -1630.70 457.67 -32.11
fitted(m)
# 1] -0.22087053 -0.09989926 -0.13579690 -0.21936385 -0.34761742 -0.50048799
# [7] -0.69729602 -0.90471194 -1.20692552 -1.61994657
FPG <- function(pH) -1630.70 + 457.67*pH -32.11*pH^2
FPG(aedf$pH)
# [1] -0.016359649 0.107602395 0.074773375 -0.007166685 -0.133786467
# [6] -0.285187665 -0.480463769 -0.686513537 -0.987013685 -1.398026917
# So why aren't fitted(m) and FPG(aedf$pH) not closer ("equal")?
This email has been scanned by BullGuard antivirus protection.
For more info visit www.bullguard.com
<http://www.bullguard.com/tracking.aspx?affiliate=bullguard&buyaffiliate=smt
p&url=/>
[[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.
