[R-meta] Some questions: effect sizes, heterogeneity and Egger's test

[Teresa Luther]

Dear All,

I conducted several meta-analyses. I used the R package "metafor" as well as the free open-source software OpenMetaAnalyst for conducting the analyses.

Now, I am writing up the results and face some uncertainty with regard to the statistics.

1) I have a p-value of .000 for several effect sizes (Hedge's g). Also for Higgins' I^2 (heterogeneity) I sometimes get this value for p.
I would tend to write p < .001 and possibly footnote the .000. Is this possible or what would you suggest in such a case? Simply report it as p < .001?

2) As a measure of heterogeneity, I interpret Higgins' I^2. I interpret the values above 25% as low heterogeneity (following Higgins et al. 2003). In some of the analyses, I get a value of 0%. I would now have to write that there is no heterogeneity. However, I consider this value almost impossible, since there is always a certain variance between the studies. Since I had assumed heterogeneity between the studies, I had also performed the calculations on the basis of a random-effects model.
I am also not sure whether values of 25, 50 and 75 % have to be considered as cut-off values or whether values in between can also be interpreted as for example "small to medium" for I^2=45 %.
This question also arises for me with Hedge's g.

3) To investigate a possible publication bias, I ran Egger's regression test in R for some of the analyses. I would like to report the regression intercept as well and assume that the intercept is the “b“ (provided together with CI) I get in the output. Would it be correct to report this value as beta^ with index 1? In the literature I found that in most meta-analyses only report the p-value for Egger's test, however the recommendation is made to report the intercept as well. Now I would like to report this regression intercept correctly.


I would be very grateful if you could help me out with my questions. (Unfortunately,) I am the first to conduct a meta-analysis in my lab and this topic has not been covered in my studies so far.
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