Confidence interval for sum of coefficients
On 14-09-25 10:17 AM, Doogan, Nathan wrote:
Is there an issue with using the variance sum law and the var-covar matrix to sum two parameters and estimate the variance of the sum? i.e., add their variances and covariances as expressed in the variance covariance matrix of the parameter estimates, probably obtained with vcov(modelObj). Or is this too simplistic for a mixed model? -Nate -- Nathan J. Doogan, Ph.D. | College of Public Health Post-Doctoral Researcher | The Ohio State University
That was exactly what I was going to suggest (but hadn't gotten around
to it). It's slightly less accurate than parametric bootstrapping or
likelihood profiling (the former is computationally straightforward, the
latter would have to be implemented more or less from scratch), but
should be fine in many cases.
To be more specific, if you have a linear combination of parameters in
mind (e.g. lincomb <- c(1,1,1) for adding all three parameters), you want
lincomb %*% vcov(fitted_model) %*% lincomb
(R should take care of the transposition where necessary, I think)
to get the variance.
By the way, I don't think it makes any sense at all to add confidence
intervals; as one example, imagine that two quantities have estimated
values of 1 and 2 with confidence intervals {-1,3} and {1,3}; should the
net confidence intervals actually be {0,6} ... ? Or add many values
with lower bounds at zero -- should the joint lower bound really be
zero? If you want to add something, add *variances* and convert to std
errors and from there to CIs ...
-----Original Message----- From: r-sig-mixed-models-bounces at r-project.org [mailto:r-sig-mixed-models-bounces at r-project.org] On Behalf Of Michael Cone Sent: Thursday, September 25, 2014 9:26 AM To: r-sig-mixed-models at r-project.org Subject: Re: [R-sig-ME] Confidence interval for sum of coefficients Dear list, Lorenz has pointed out to me Ben's suggestion to bootstrap the sums (or any linear combiantion) of coefficients I'm interested in. This may be the general approach, but I struggle to see why it would be illegitimate to simply change the reference level for the treatment contrast coding, fit the model again and run confint() a second time (and do so again for MachineC):
Machines$Machine <- relevel(Machines$Machine, 'B') fm2 <- lmer(score ~ Machine + (Machine | Worker), data = Machines) summary(fm2)
Fixed effects: Estimate Std. Error t value (Intercept) 60.322 3.529 17.096 MachineA -7.967 2.421 -3.291 MachineC 5.950 2.446 2.432
confint(fm2)
(Intercept) 52.8500103 67.7944456 MachineA -13.0931710 -2.8401544 MachineC 0.7692323 11.1307757 Now the CI of the intercept is the confidence interval for the overall score of MachineB. Adding lower and upper bounds from fm1 would have given somewhat similar, but somewhat wider intervals. (I probably have a lack of understanding as to how CIs can be calculated with. Is there an inuitive explanation for why the bounds don't add?)
Machines$Machine <- relevel(Machines$Machine, 'C') fm3 <- lmer(score ~ Machine + (Machine | Worker), data = Machines) summary(fm3)
Fixed effects: Estimate Std. Error t value (Intercept) 66.272 1.806 36.69 MachineB -5.950 2.446 -2.43 MachineA -13.917 1.540 -9.04
confint(fm3)
(Intercept) 62.4471752 70.0972752 MachineB -11.1307677 -0.7692243 MachineA -17.1780524 -10.6552759 Thanks, and best wishes Michael Am 25.09.2014 14:11 schrieb Michael Cone:
Hello, I suspect this to be simple, but I can't figure it out.
library(lme4) data(Machines) fm1 <- lmer(score ~ Machine + (Machine | Worker), data = Machines) summary(fm1)
Fixed effects: Estimate Std. Error t value (Intercept) 52.356 1.681 31.151 MachineB 7.967 2.421 3.291 MachineC 13.917 1.540 9.036
confint(fm1)
2.5 % 97.5 % [...] (Intercept) 48.7964047 55.9147119 MachineB 2.8401623 13.0931789 MachineC 10.6552809 17.1780575 [and 14 warnings, but it's just an example: In optwrap(optimizer, par = start, fn = function(x) dd(mkpar(npar1, ... : convergence code 1 from bobyqa: bobyqa -- maximum number of function evaluations exceeded ... In profile.merMod(object, signames = oldNames, ...) : non-monotonic profile] I'd like to have confidence intervals for the overall score of MachineA, MachineB, and MachineB. MachineA is easy (CI of the intercept), but how do I combine the CI of the intercept with the CI of the MachineB parameter, and likewise the CI of the intercept with the parameter of MachineC? Can I simply add the lower and upper bounds of the two intervals or is this naive? Thank you for your time, Michael
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