[R-sig-ME] Set two coefficients to be equal in lme()

[Simon Harmel]

Dear James,

Thank you for your answer. I have a follow-up question. In my `dat` dataset
below, "Treat" is coded {-.5, .5}. D1 & D2 are simply dummy coded {0, 1} .

My question is: why when I recode "Treat" as a factor {-.5="C", .5="T"},
then the model fails? Below is a reproducible example:

library(nlme)

(dat <- read.csv("https://raw.githubusercontent.com/hkil/m/master/mv.l.csv
"))

model_works <- lme(value ~ 0 + D1 + D2+D1:Treat+D2:Treat, random = ~0 + D1
+ D2 | Student, data = dat)

(dat2 <- transform(dat, Treat = ifelse(Treat==.5,"T", "C")))  # recode
Treat as a factor

model_fails <- lme(value ~ 0 + D1 + D2+D1:Treat+D2:Treat, random = ~0 + D1
+ D2 | Student, data = dat2)

On Sat, Jan 16, 2021 at 12:10 AM Uanhoro, James <
uanhoro.1 using buckeyemail.osu.edu> wrote:

> Dear Simon,
>
> Create a variable that is the sum of D1 * Treat and D2 * Treat and place
> it in the model:
>
> data$new_var <- (data$D1 + data$D2) * data$Treat
> lme(value ~ 0 + D1 + D2 + new_var, random = ~0 + D1 + D2 | Student, data =
> data)
>
> The coefficient of new_var in the model above is the coefficient for both
> variables if their coefficients were constrained equal.
>
> James.
>
> On Fri, 2021-01-15 at 23:42 -0600, Simon Harmel wrote:
> Dear All,
>
> I was wondering if it might be possible in my below model to set
> "D1:Treat" and "D2:Treat" to be equal?
>
> lme(value ~ 0 + D1 + D2 + D1:Treat + D2:Treat, random = ~0 + D1 + D2 |
> Student, data = data)
>
> Thank you,
> Simon
>
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