Dear Jeff, I am not sure if I understood the procedure properly but it looks like it works: ``` Y <-c(1.301030, 1.602060, 1.903090, 2.204120, 2.505150, 2.806180, 3.107210, 3.408240, 3.709270, 4.010300, 4.311330, 4.612360, 4.913390, 5.214420, 5.515450, 5.816480, 6.117510, 6.418540, 6.719570, 7.020599, 7.321629, 7.622658, 7.923686, 8.224713, 8.525735, 8.826751, 9.127752, 9.428723, 9.729637, 10.030434, 10.330998, 10.631096, 10.930265, 11.227580, 11.521213, 11.807577, 12.079787, 12.325217, 12.523074, 12.647915, 12.693594, 12.698904, 12.698970, 12.698970, 12.698970) X <- 1:45 plot(Y~X) raw_value <- predict(lm(X[1:39]~Y[1:39]), newdata = data.frame(Y=6)) x <- unname(raw_value[!is.na(raw_value)]) # x= 16.62995 points(x, 6, pch = 16) ``` Here I used the points 1:39 because afterward there is a bend. But I am not clear why I need to use `lm(X~Y),` instead of `lm(Y~X)`. Thank you
On Tue, Jan 26, 2021 at 10:20 AM Jeff Newmiller <jdnew...@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.lu...@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 > > > >______________________________________________ > >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. -- Best regards, Luigi ______________________________________________ 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.
