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c980
2011 Feb 04
1
Quadratic regression: estimating the maximizing value
...} # Solve d/dx(A + Bx + Cx^2) = 0
> DZ(LM$coefficients[2],LM$coefficients[3])
X
13.20961
To find a confidence interval, I used "confint()".
Default confidence level of 95% was not useful; used 80% instead,
and then computed DZ with the extreme X and I(X^2) coefficients:
>(CI80<-confint(LM,level=0.8))
10 % 90 %
(Intercept) -5.6147948 -2.12476138
X 1.4476460 2.10759306
I(X^2) -0.0790312 -0.05553898
> DZ(CI80[2,1],CI80[3,1])
[1] 9.1587
> DZ(CI80[2,2],CI80[3,2])
[1] 18.97400
Conclusion: the 80% confidence level for the ma...