[R] Intercept of confidence interval with a constant

From: Larry Howe <linux_at_comjet.com>
Date: Sat 03 Jun 2006 - 00:52:43 EST


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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