[R-meta] "Categorical" moderator varying within and between studies

[Simon Harmel]

Many thanks, James! I keep getting the following error when I run your code:

Error: unexpected symbol in:
"Rating" = "Relative")
oswald_centered"

On Tue, Jun 2, 2020 at 10:00 PM James Pustejovsky <jepusto using gmail.com> wrote:

> Hi Simon,
>
> The same strategy can be followed by using dummy variables for each unique
> level of a categorical moderator. The idea would be to 1) create dummy
> variables for each category, 2) calculate the study-level means of the
> dummy variables (between-cluster predictors), and 3) calculate the
> group-mean centered dummy variables (within-cluster predictors). Just like
> if you're working with regular categorical predictors, you'll have to pick
> one reference level to omit when using these sets of predictors.
>
> Here is an example of how to carry out such calculations in R, using the
> fastDummies package along with a bit of dplyr:
>
> library(dplyr)
> library(fastDummies)
> library(robumeta)
>
> data("oswald2013")
>
> oswald_centered <-
>   oswald2013 %>%
>
>   # make dummy variables
>   mutate(
>     Scoring = recode(Scoring, "Difference Score" = "Difference", "Relative
> Rating" = "Relative")
>   ) %>%
>   dummy_columns(select_columns = "Scoring") %>%
>
>   # centering by study
>   group_by(Study) %>%
>   mutate_at(vars(starts_with("Scoring_")),
>             list(wthn = ~ . - mean(.), btw = ~ mean(.))) %>%
>
>   # calculate Fisher Z and variance
>   mutate(
>     Z = atanh(R),
>     V = 1 / (N - 3)
>   )
>
>
> # Use the predictors in a meta-regression model
> # with Scoring = Absolute as the omitted category
>
> robu(Z ~ Scoring_Difference_wthn + Scoring_Relative_wthn +
> Scoring_Difference_btw + Scoring_Relative_btw, data = oswald_centered,
> studynum = Study, var.eff.size = V)
>
>
> Kind Regards,
> James
>
> On Tue, Jun 2, 2020 at 6:49 PM Simon Harmel <sim.harmel using gmail.com> wrote:
>
>> Hi All,
>>
>> Page 13 of *THIS ARTICLE
>> <
>> https://cran.r-project.org/web/packages/robumeta/vignettes/robumetaVignette.pdf
>> >*
>>  (*top of the page*) recommends that if a *continuous moderator *varies
>> both within and across studies in a meta-analysis, a strategy is to break
>> that moderator down into two moderators by:
>>
>> *(a)* taking the mean of each study (between-cluster effect),
>>
>> *(b)* centering the predictor within each study (within-cluster effect).
>>
>> BUT what if my original moderator that varies both within and across
>> studies is a *"categorical" *moderator?
>>
>> I appreciate an R demonstration of the strategy recommended.
>> Thanks,
>> Simon
>>
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>>
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