Dear R-experts!

Running a meta-analysis (using the magnificent metafor-package), I use
cohen's d as a main outcome measure in a random-effects model.
For most of the samples cohen's d is derived form a comparison of two groups
(A & B). However some studies report results from an ANOVA (one-factor with
three levels: C,D,E) whereas two groups (C,D) correspond to one group in the
other studies (B=C,D). Is there a way?
The handbook of research synthesis and meta-analysis By Harris M. Cooper
says that:
d=sqrt((F*(n1+n2)/n1*n2)) for ANOVA (one-factor with two-levels)
but does this also hold for ANOVA (one-factor with three levels)?

Many thanks for your help!
Jokel

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