Re: [R] Intercept of confidence interval with a constant

From: Uwe Ligges <>
Date: Sat 03 Jun 2006 - 01:35:34 EST

Larry Howe wrote:

> Hello,
> Is there a way for R to determine the point where a confidence interval equals
> a specified value? For example:
> x = seq(1:5)
> y = c(5, 5, 4, 4, 3)
> lm = lm(y ~ x)
> p = predict.lm(lm, interval="confidence")
> matplot(p, type="b")
> abline(h = 3)

You can do it this way:


            (predict(lm, newdata = data.frame(x=x),
                     interval = "confidence")[,2] - 3)^2,
           interval=c(1, 5))$minimum

but maybe you are going to solve a "calibration" problem, in fact and a completely different kind of confidence interval (namely for the x-axis)?

Uwe Ligges

> I want to answer the question: "What is the value of x when the y-value of the
> lower confidence interval is equal to 3.0"? Visually, it is the place on the
> example where the abline intersects the lower confidence interval, or about
> 4.2. Can R calculate this number for me?
> I know that predict.lm will calculate a y-value from an x-value, but what I
> want is the opposite. I know the y-value, and I want to calculate the
> x-value.
> Larry Howe
> ______________________________________________
> mailing list
> PLEASE do read the posting guide! mailing list PLEASE do read the posting guide! Received on Sat Jun 03 01:48:41 2006

Archive maintained by Robert King, hosted by the discipline of statistics at the University of Newcastle, Australia.
Archive generated by hypermail 2.1.8, at Sat 03 Jun 2006 - 04:10:30 EST.

Mailing list information is available at Please read the posting guide before posting to the list.