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
[R-meta] Impact of very high tau-square value.
3 messages · jideofor thomas, Gerta Ruecker, Guido Schwarzer
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. Best, Gerta 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 R?cker, 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 at imbi.uni-freiburg.de Homepage: https://www.imbi.uni-freiburg.de/persons/ruecker/person_view -------------- next part -------------- A non-text attachment was scrubbed... Name: RuckerSchwarzerCarpenterSchumacherI2BMC2008.pdf Type: application/pdf Size: 325306 bytes Desc: not available URL: <https://stat.ethz.ch/pipermail/r-sig-meta-analysis/attachments/20190626/c7aaac76/attachment-0001.pdf>
Dear Jide Ogbu, It is not possible to give any specific advice on your dataset as you do not provide sufficient information. So, I will make some general comments. A plethora of methods to estimate tau-squared is available (Veroniki et al., 2016) and they can give different estimates; see argument 'method' in metafor. Furthermore, whether a tau-squared value of 32.8 is large depends on the outcome measure (and for some outcomes on the underlying metric). This value could be judged to be small if you are interested in a mean difference - depending on your metric. However, if your outcome measure is a standardized mean difference or a log odds ratio a tau-squared value of 32.8 would be very extreme. Best wishes, Guido Veroniki AA, Jackson D, Viechtbauer W, Bender R, Bowden J, Knapp G, et al. Methods to estimate the between-study variance and its uncertainty in meta-analysis. Research Synthesis Methods. 2016;7:55?79.