Home forums Mixed Models Including a covariate

This topic contains 1 reply, has 2 voices, and was last updated by  henrik 4 weeks ago.

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  • #347

    Melvin Sim
    Participant

    Hello, I want to ask how do I include a co-variate in the model when using the aov-car function.
    Below is the syntax I have created, I am running a mixed model with time as a within variable and condition as a between variable:
    fit.grat2 <-aov_car(grat ~ condition*time + Error(PPT/time),
    data=mf_long2,
    covariate=centgrat)

    I have specified that centered trait gratitude (centgrat) is meant to be a covariate. My question is whether I have to include it in the actual model like so instead:
    fit.grat2 <-aov_car(grat ~ tgrat+condition*time + Error(PPT/time),
    data=mf_long2,
    covariate=centgrat)

    • This topic was modified 1 month, 2 weeks ago by  Melvin Sim. Reason: For clarity
  • #351

    henrik
    Keymaster

    Sorry for the slow reply, your post somehow went through the cracks. You need to set factorize = FALSE in the call to use numerical covariates.

    For the example data that would be:

    data(obk.long, package = "afex")
    aov_ez("id", "value", obk.long, between = c("treatment", "gender"), 
            within = c("phase", "hour"), covariate = "age", 
            observed = c("gender", "age"), factorize = FALSE)

    In your case this would be:

    fit.grat2 <-aov_car(grat ~ tgrat+condition*time + Error(PPT/time),
    data=mf_long2, covariate=centgrat, factorize = FALSE)

    I should probably add a corresponding note to the documentation.

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