[R] Intercept of confidence interval with a constant
Larry Howe
linux at comjet.com
Fri Jun 2 16:52:43 CEST 2006
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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