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

From: Uwe Ligges <ligges_at_statistik.uni-dortmund.de>
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:

optimize(function(x)

```            (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
>
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