At 16:38 27/08/2014, Owen, Branwen wrote:
Thank you very much for your quick reply Wolfgang. The 0 does make
sense - I'm working on some behavioural data and in one study none
reported that particular behaviour. Up til now I have been
calculating I2 without that study, as it's on the small side I don't
think it makes much difference. I'm just beginning to wonder if
there is another way.
Branwen
I have not tried this but if your dataset contains proportions as
your text implies then why not supply xi and ni to rma.uni and then
get it to compute one of the range of options outlined in the
documentation for escalc? The Freeman-Tukey one might be worth considering.
Of course I may have misunderstood your brief description.
I'll think about it some more best wishes Branwen
________________________________________ From: Viechtbauer Wolfgang
(STAT) [wolfgang.viechtba...@maastrichtuniversity.nl] Sent: 27
August 2014 17:30 To: Owen, Branwen; r-help@r-project.org Subject:
RE: [R] Metafor -can't calculate heterogeneity with non-positive
sampling variances The warning message pretty much says it: When one
of the variances is zero, then the I^2 statistic (and various other
things) cannot be computed, at least if one sticks to the usual
equations/methods. So, if you think the 0 sampling variances really
make sense and you really want to get something like I^2, you will
have to come up with a creative solution. On the metafor package
website, I explain how I^2 is computed (for the random-effects
model):
http://www.metafor-project.org/doku.php/faq#how_are_i_2_and_h_2_computed_i
The crux of the problem is how to compute the 'typical' within-study
variance (s^2). With any vi=0, you get division by zero in the
equation given. So, you will have to compute s^2 in a different way.
You could leave out the studies where vi=0, but this doesn't seem
quite right, because this will inflate s^2. You could just take the
simple average of the vi values and use that for s^2, but then it's
not really I^2 anymore (it's I^2-like). My question would be: How
come you have studies where the sampling variance is estimated to be
zero and does that really make sense? Maybe the solution is not to
fix the computation of I^2, but to consider if vi=0 is really
sensible. Best, Wolfgang > -----Original Message----- > From:
r-help-boun...@r-project.org [mailto:r-help-boun...@r-project.org] >
On Behalf Of Owen, Branwen > Sent: Wednesday, August 27, 2014
13:48 > To: r-help@r-project.org > Subject: [R] Metafor -can't
calculate heterogeneity with non-positive > sampling variances > >
Hi, I'm doing a meta-analysis in metafor. All is fine except when
there > are 0s in the values that i'm pooling, then i get a pooled
estimate but > not the I2 that i am also interested in. > for
example: > > summary(rma.1<- >
rma(yi,vi,data=mix,method="ML",knha=F,weighted=F,intercept=T)) >
(where yi are the study outcomes, one of which is 0, and vi is the >
variance of the study outcomes) > > Random-Effects Model (k = 17;
tau^2 estimator:
ML) > > logLik deviance AIC BIC AICc > 13.0539
Inf -22.1077 -20.4413 -21.2506 > > tau^2 (estimated amount
of total heterogeneity): 0.0119 (SE = 0.0043) > tau (square root of
estimated tau^2 value): 0.1089 > > Model Results: > >
estimate se zval pval ci.lb ci.ub > 0.1837
0.0274 6.7154 <.0001 0.1301 0.2374 *** > > --- >
Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1 > >
Warning messages: > 1: In rma(yi, vi, data = mix, method = "ML",
knha = F, weighted = F, : > There are outcomes with non-positive
sampling variances. > 2: In rma(yi, vi, data = mix, method = "ML",
knha = F, weighted = F, : > Cannot compute Q-test, I^2, or H^2
with non-positive sampling > variances. > > Is there any way around
this? > thanks > Branwen >
________________________________________ > From:
r-help-boun...@r-project.org [r-help-boun...@r-project.org] on >
behalf of r-help-ow...@r-project.org [r-help-ow...@r-project.org] >
Sent: 27 August 2014 13:36 > To: Owen, Branwen > Subject: Metafor
-can't calculate heterogeneity with non-positive > sampling
variances > > Message rejected by filter rule match > > >
______________________________________________ >
R-help@r-project.org mailing list >
https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the
posting guide http://www.R-project.org/posting- > guide.html > and
provide commented, minimal, self-contained, reproducible code.
Michael Dewey
i...@aghmed.fsnet.co.uk
http://www.aghmed.fsnet.co.uk/home.html
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