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

From: Larry Howe <linux_at_comjet.com>
Date: Sat 03 Jun 2006 - 06:46:56 EST

Ah, so I switch y with x, give it x, and ask it for y (which is actually x). Clever.

I continue to be impressed with R.

Thanks, Larry

On Friday June 2 2006 11:23, Gabor Grothendieck wrote:
> Try this:
>
> approx(p[,"lwr"], 1:5, 3)
>
> On 6/2/06, Larry Howe <linux@comjet.com> 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)
> >
> > 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
> >
> > ______________________________________________
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