Access results of intervals.lme as a dataframe + r + lme

April 4, 2023

I am using a linear mixed effects model to determine the relationship between 2 variables over time, model –

data(mtcars)

# linear mixed effects model
mlme <- lme(mpg ~ wt, random = ~ 1|cyl, data = mtcars)

I then access the confidence intervals using –

conf <- intervals(mlme, level = 0.95, which = 'fixed')
conf

Returning the results –

Approximate 95% confidence intervals

 Fixed effects:
               lower      est.    upper
(Intercept) 7.431921 18.416639 29.40136
DairyTotal  2.001397  6.716849 11.43230

I want to access the values of the lower and upper limits so I can use them as variables but can’t find a method to do so. If you want to access the coefficients for example you can use coef(summary(mlme)) and convert it to a dataframe.

I tried converting the result to a dataframe using data.frame(coef) but get the error:

> data.frame(conf)
Error in as.data.frame.default(x[[i]], optional = TRUE, stringsAsFactors = stringsAsFactors) : 
  cannot coerce class ‘"intervals.lme"’ to a data.frame

Can anyone recommend a method to access variables that are returned in this format?

>Solution :

Here is an example with the mtcars dataset how we could access the lower and upper ci as variables:

library(nlme)

data(mtcars)

# linear mixed effects model
mlme <- lme(mpg ~ wt, random = ~ 1|cyl, data = mtcars)

# Confidence intervals
conf <- intervals(mlme, level = 0.95, which = 'fixed')

# Lower and upper confidence intervals as variables
lower <- conf$fixed["(Intercept)", "lower"]
upper <- conf$fixed["(Intercept)", "upper"]

[1] 25.7204
[1] 37.32919