[R-meta] Impact of very high tau-square value.

Gerta Ruecker ruecker @end|ng |rom |mb|@un|-|re|burg@de
Wed Jun 26 11:27:19 CEST 2019

Dear Jide,

in principle, tau -squared can take every non-negative value. Note that 
tau is the standard deviation of the random effects. That is, it is 
measured on the same scale as the outcome and has the same dimension. 
That is, if you measure blood pressure, it is on the scale of blood 
pressure, mmHg; if you measure body weight, it is on the scale of body 
weight, i.e. kg. There is no other general rule.

If you look at a dimensionless outcome (e.g., (log) odds ratio or 
standardized mean difference), the same holds: tau is on the log odds 
scale or on the normal scale, respectively.

Thus the meaning of your values depends on what you are measuring.

See the attached paper, page 7.



Am 26.06.2019 um 09:32 schrieb jideofor thomas:
> Hi all,
> I did a random effect meta-analysis using the metafor package and I obtained a tau-squared ( heterogeneity)  value greater than 1.
> I was worried because I am nit sure what to make of it; so I did the same analysis under the Bayesian framework. I also obtained a tau-squared value of about 32.8.
> I find it confusing, for one it is quite high and secondly I hadn't come across any discussion on how high the tau-squared value could get.
> My questions are:
> How high can the tau-squared value  be?
> Secondly, under this situation can I go ahead and interpret my result but with a warning on the amount of heterogeneity present in the study?  Or does the high value of the tau-square make non-sense of my analysis?
> Thanks in advance for your response.
> Jide Ogbu
> 	[[alternative HTML version deleted]]
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Dr. rer. nat. Gerta Rücker, Dipl.-Math.

Institute of Medical Biometry and Statistics,
Faculty of Medicine and Medical Center - University of Freiburg

Stefan-Meier-Str. 26, D-79104 Freiburg, Germany

Phone:    +49/761/203-6673
Fax:      +49/761/203-6680
Mail:     ruecker using imbi.uni-freiburg.de
Homepage: https://www.imbi.uni-freiburg.de/persons/ruecker/person_view

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