[R] nls problem

[Rui Barradas]

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")?
> 
>   
> 
> 
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