R - Pass column names as Variable with names contain I()

Question

I'm performing the polynomial regression and testing the linear combination of the coefficient. But I'm running to some problems that when I tried to test the linear combination of the coefficient.

LnModel_1 <- lm(formula = PROF ~ UI_1+UI_2+I(UI_1^2)+UI_1:UI_2+I(UI_2^2)) 
summary(LnModel_1)

It output the values below:

Call:
lm(formula = PROF ~ UI_1 + UI_2 + I(UI_1^2) + UI_1:UI_2 + I(UI_2^2))
Residuals:
 Min      1Q  Median      3Q     Max 
-3.4492 -0.5405  0.1096  0.4226  1.7346 
Coefficients:
Estimate Std. Error t value Pr(>|t|)    
(Intercept)  4.66274    0.06444  72.354  < 2e-16 ***
UI_1         0.25665    0.07009   3.662 0.000278 ***
UI_2         0.25569    0.09221   2.773 0.005775 ** 
I(UI_1^2)   -0.15168    0.04490  -3.378 0.000789 ***
I(UI_2^2)   -0.08418    0.05162  -1.631 0.103643    
UI_1:UI_2   -0.02849    0.05453  -0.522 0.601621

Then I use names(coef()) to extract the coefficient names

names(coef(LnModel_1))
output:
[1] "(Intercept)" "UI_1"        "UI_2"        "I(UI_1^2)" 
"I(UI_2^2)""UI_1:UI_2"

For some reasons, when I use glht(), it give me an error on UI_2^2

slope <- glht(LnModel_1, linfct = c("UI_2+ UI_1:UI_2*2.5+ 2*2.5*I(UI_2^2) =0
") )
Output:
Error: multcomp:::expression2coef::walkCode::eval: within ‘UI_2^2’, the term
‘UI_2’ must not denote an effect. Apart from that, the term must evaluate to
a real valued constant

Don't know why it would give me this error message. How to input the I(UI_2^2) coefficient to the glht()

Thank you very much

Suggest you use [edit] facilities to add [MCVE] – IRTFM Mar 09 '19 at 06:32 — IRTFM, Mar 09 '19 at 06:32

score 0 · Accepted Answer · answered Mar 09 '19 at 09:52

The issue seems to be that I(UI^2) can be interpreted as an expression in R in the same fashion you did here LnModel_1 <- lm(formula = PROF ~ UI_1+UI_2+I(UI_1^2)+UI_1:UI_2+I(UI_2^2))

Therefore, you should indicate R that you want to evaluate a string inside your string:

slope <- glht(LnModel_1, linfct = c("UI_2+ UI_1:UI_2*2.5+ 2*2.5*\`I(UI_2^2)\` =0
") )

Check my example (since I cannot reproduce your problem):

library(multcomp)

cars <- copy(mtcars)
setnames(cars, "disp", "UI_2")
model <- lm(mpg~I(UI_2^2),cars)

names(coef(model))

slope <- glht(model, linfct = c("2*2.5*\`I(UI_2^2)\` =0") )

R - Pass column names as Variable with names contain I()

1 Answers1