[R-meta] metamean

Fri, Mar 10, 2023 6:12 AM

The message is pretty clear: Studies where the SD is 0 are omitted.

Compare:

metamean(mean=c(2,3,4), sd=c(1,1,1), n=c(50,50,50), comb.random=TRUE, comb.fixed=FALSE)
metamean(mean=c(2,3,4), sd=c(1,1,0), n=c(50,50,50), comb.random=TRUE, comb.fixed=FALSE)

In the second case, the third study is omitted because its SD is 0.

So, I think you need to tripple check your data.

Best,
Wolfgang

-----Original Message-----
From: R-sig-meta-analysis [mailto:r-sig-meta-analysis-bounces at r-project.org] On
Behalf Of mark walton via R-sig-meta-analysis
Sent: Friday, 10 March, 2023 15:00
To: r-sig-meta-analysis at r-project.org
Cc: mark walton
Subject: [R-meta] metamean

Dear Michael

Apologies, I thought i had started a new thread.

I have checked the SDs and omitted any that are zero, so I don?t think that is
the problem. I am more worried by the warning ?Studies with non-positive standard
deviation get no weight in meta-analysis? I assume this signifies that means with
a standard deviations of greater value than the mean are ignored (eg mean ? SD =
10 ? 14). Is that correct?

Thanks
Mark

On Friday, 10 March 2023 at 14:24:55 CET, Michael Dewey <lists at dewey.myzen.co.uk>
wrote:

Dear Mark

First of all please do not hijack an old thread but start a new one as
your e-mail (and my reply) appear to be about marginal means in the archive.

Have you checked the standard deviations? I assume some are zero which
explains both your messages. It is likely the package then drops them.

Michael

On 10/03/2023 11:46, mark walton via R-sig-meta-analysis wrote:

? Dear All
I wonder if some one can help.
I am using metamean to determine with of my habitats have the most

microplastics, I have collected 400+ papers with means, SD and sample
number.Using the command:-metamean(MP_global, mean=meanMP_per_m2, na.rm=TRUE,
sd=MP_SD, na.rm=TRUE,n=Sample_no, na.rm=TRUE ,byvar=Ecosystem,
na.rm=TRUE,comb.random=TRUE,comb.fixed=FALSE,verbose=TRUE,
control=list(maxiter=1000))

I get some repeated warnings:?Warning: Studies with non-positive standard

deviation get no weight in meta-analysis.Warning: Ratio of largest to smallest
sampling variance extremely large. May not be able to obtain stable results.

But the model converges and I get means and confidence intervals, and

significant differences between subgroups

However when I estimate my own mean values for each habitat weighted by sample

number (below) the values are very different for some, and I am worried that the
metamean results are not a true reflection of the data.

ddply(MP_global, .(Ecosystem),? function(x)

data.frame(wret=weighted.mean(x$meanMP_per_m2, x$Sample_no)))

Can anyone shed some light? Is it because metamean is ignoring some values?
Thanks very muchMark

[R-meta] metamean

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