[R-meta] meta-regression: categorical

[P. Roberto Bakker]

Hi Everybody,

I have a new question about meta-regression.
My database consists only of factors (no characters), so I suppose I do not
need 'factor()' in 'mods', i.e. 'mods = ~ factor-var'. I also tried 'mods =
~ factor(factor-var)' and I receive the same results as expected.
Only, there is one difference, 'mods = ~ factor-var' gives a warning
message:
*"In rma(measure = "SMCC", yi = yi, vi = vi, data = datsub, digits = 2,  :*
*  Redundant predictors dropped from the model."*
Whereas 'mods = ~ factor(factor-var)' does not give this message. What is
the reason for this?

So, my question is not why I receive this message, but why the difference:
with/out warning message.

Best regards,
Roberto
PS the same happens with a character variable: i.e. 'mods = ~ chr-var' as
well as 'mods = ~ factor(chr-var)'.

Op do 26 apr. 2018 om 07:42 schreef P. Roberto Bakker <
robertobakker using gmail.com>:

> Hi Wolfgang,
>
> Thank you for your information and explanation.
>
> Bw Roberto
>
> 2018-04-24 12:25 GMT+02:00 Viechtbauer, Wolfgang (SP) <
> wolfgang.viechtbauer using maastrichtuniversity.nl>:
>
>> Hi Roberto,
>>
>> If you put a character variable (or a factor) into a formula, it is
>> automatically dummy coded. Actually, the type of coding depends on:
>>
>> options("contrasts")
>>
>> But the default for that is:
>>
>>         unordered           ordered
>> "contr.treatment"      "contr.poly"
>>
>> And help(contr.treatment) explains what kind of coding this is, namely
>> the 'usual' dummy coding where one level is the 'reference' level (and
>> whose dummy gets omitted).
>>
>> Best,
>> Wolfgang
>>
>> -----Original Message-----
>> From: R-sig-meta-analysis [mailto:
>> r-sig-meta-analysis-bounces using r-project.org] On Behalf Of P. Roberto Bakker
>> Sent: Tuesday, 24 April, 2018 11:14
>> To: r-sig-meta-analysis using r-project.org
>> Subject: [R-meta] meta-regression: categorical
>>
>> Hi everybody,
>>
>> In:
>> http://www.metafor-project.org/doku.php/analyses:vanhouwelingen2002
>> 'allocation' is put in syntax without dummy coding
>>
>> rma <http://finzi.psych.upenn.edu/library/metafor/html/rma.uni.html>(yi,
>> vi, mods = ~ alloc, data
>> <http://stat.ethz.ch/R-manual/R-devel/library/utils/html/data.html>=dat,
>> method="ML")
>>
>> If I am not wrong, 'allocation' is a catergorical variabel, and I suppose
>> the R handles the coding of dummy automatically.
>> Correct?
>>
>> I found this information in:
>> https://cran.r-project.org/web/packages/metafor/metafor.pdf
>> *Categorical moderator variables can be included in the model via the mods
>> argument in the same way that appropriately (dummy) coded categorical
>> independent variables can be included in linear models. One can either do
>> the dummy coding manually or use a model formula together with the factor
>> function to let R handle the coding automatically.   *
>>
>> *Best wishes and thank you in advance*
>> *Roberto*
>>
>
>

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