[R] How to predict/interpolate new Y given knwon Xs and Ys?

[Luigi Marongiu]

I see, so predict is mono-directional: only gives x with a know y but
not the other way round. Thank you

On Tue, Jan 26, 2021 at 10:20 AM Jeff Newmiller
<jdnewmil using dcn.davis.ca.us> wrote:
>
> model2 <- lm( x~y )
> predict(model2, data.frame(y=26))
>
> model2 is however not the inverse of model... if you need that then you need to handle that some other way than using predict, such as an invertible monotonic spline (or in this case a little algebra).
>
> On January 26, 2021 1:11:39 AM PST, Luigi Marongiu <marongiu.luigi using gmail.com> wrote:
> >Hello,
> >I have a series of x/y and a model. I can interpolate a new value of x
> >using this model, but I get funny results if I give the y and look for
> >the correspondent x:
> >```
> >> x = 1:10
> >> y = 2*x+15
> >> model <- lm(y~x)
> >> predict(model, data.frame(x=7.5))
> > 1
> >30
> >> predict(model, data.frame(y=26))
> > 1  2  3  4  5  6  7  8  9 10
> >17 19 21 23 25 27 29 31 33 35
> >Warning message:
> >'newdata' had 1 row but variables found have 10 rows
> >> data.frame(x=7.5)
> >    x
> >1 7.5
> >> data.frame(y=26)
> >   y
> >1 26
> >```
> >what is the correct syntax?
> >Thank you
> >Luigi
> >
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> --
> Sent from my phone. Please excuse my brevity.



-- 
Best regards,
Luigi