[R] Metafor: How to integrate effectsizes?

[Viechtbauer Wolfgang (STAT)]

Without the sample size of a study (i.e., either the group sizes or the total sample size), you cannot convert the p-value to a t-value or a t-value to a d-value. And for studies where you have the d-value but no sample size, you cannot compute the corresponding sampling variance. So, without additional information, you cannot include these studies. Maybe studies where a d-value is directly reported also report a CI for the d-value? Then the sampling variance can be back-calculated (since a 95% CI for d is typically computed with d +- 1.96 sqrt(vi), where vi is the sampling variance).

Best,
Wolfgang

> -----Original Message-----
> From: Verena Weinbir [mailto:vweinbir at gmail.com]
> Sent: Tuesday, May 06, 2014 15:09
> To: Michael Dewey
> Cc: Viechtbauer Wolfgang (STAT); r-help at r-project.org
> Subject: Re: [R] Metafor: How to integrate effectsizes?
> 
> Thank you very much for your illustration, Wolfgang! It helped me a
> lot.  And also thank you for the package-hint, Michael!
> 
> Now, I have re-checked the respective studies, and there still are a
> couple of studies left, only stating cohens d, and the respective t-value
> and p-value - sample and group sizes are not addressed (its data from an
> older meta-analysis). Is there a way to embed these studies in my sample?
> Wolfgangs illustration addresses only cases in which group sizes are
> stated, if I understand you correctly...
> 
> Many thanks in advance,
> 
> Verena
> 
> On Sat, Apr 26, 2014 at 1:38 PM, Michael Dewey <info at aghmed.fsnet.co.uk>
> wrote:
> At 20:34 25/04/2014, Viechtbauer Wolfgang (STAT) wrote:
> If you know the d-value and the corresponding group sizes for a study,
> then it's possible to add that study to the rest of the dataset. Also, if
> you only know the test statistic from an independent samples t-test (or
> only the p-value corresponding to that test), it's possible to back-
> compute what the standardized mean difference is.
> 
> I added an illustration of this to the metafor package website:
> 
> http://www.metafor-project.org/doku.php/tips:assembling_data_smd
> 
> Verena might also like to look at the compute.es package available from
> CRAN to see whether any of the conversions programmed there do the job.
> 
> 
> Best,
> Wolfgang
> 
> --
> Wolfgang Viechtbauer, Ph.D., Statistician
> Department of Psychiatry and Psychology
> School for Mental Health and Neuroscience
> Faculty of Health, Medicine, and Life Sciences
> Maastricht University, P.O. Box 616 (VIJV1)
> 6200 MD Maastricht, The Netherlands
> +31 (43) 388-4170 | http://www.wvbauer.com
> 
> > -----Original Message-----
> > From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
> project.org]
> > On Behalf Of Michael Dewey
> > Sent: Friday, April 25, 2014 16:23
> > To: Verena Weinbir
> > Cc: r-help at r-project.org
> > Subject: Re: [R] Metafor: How to integrate effectsizes?
> >
> > At 12:33 25/04/2014, you wrote:
> > >Thank you very much for your reply and the book recommendation,
> Michael.
> > >
> > >Yes, I mean Cohen's d - sorry for the typo :-)
> > >
> > >Just to make this sure for me: There is no
> > >possibility to integrate stated Cohens' ds in an
> > >R-Metaanalysis (or a MA at all), if there is no
> > >further information traceable regarding SE or the like?
> >
> > If there is really no other information like
> > sample sizes, significance level, value of some
> > significance test then you would have to impute a
> > value from somewhere. That would seem a last resort.
> >
> > I have cc'ed this back to the list, please keep
> > it on the list so others may benefit and contribute.
> >
> >
> > >best regards,
> > >
> > >Verena
> > >
> > >
> > >On Fri, Apr 25, 2014 at 1:21 PM, Michael Dewey
> > ><<mailto:info at aghmed.fsnet.co.uk>info at aghmed.fsnet.co.uk> wrote:
> > >At 13:15 24/04/2014, Verena Weinbir wrote:
> > >Hello!
> > >
> > >I am using the metafor package for my master's thesis as an R-newbie.
> > While
> > >calculating effectsizes from my dataset (mean values and
> > >standarddeviations) using "escalc" shouldn't be a problem (I hope ;-
> )),
> > I
> > >wonder how I could at this point integrate additional studies, which
> > only
> > >state conhens d (no information about mean value and sds available),
> to
> > >calculate an overall analysis. Â I would be very grateful for your
> > support!
> > >
> > >
> > >You mean Cohen's d I think.
> > >
> > >You will need some more information to enable
> > >you to calculate its standard error. Have a look at Rosenthal's
> chapter
> > in
> > >@book{cooper94,
> > >Â  Â author = {Cooper, H and Hedges, L V},
> > >Â  Â title = {A handbook of research synthesis},
> > >Â  Â year = {1994},
> > >Â  Â publisher = {Russell Sage},
> > >Â  Â address = {New York},
> > >Â  Â keywords = {meta-analysis}
> > >}
> > >(There is an updated edition)
> > >This gives you more information about converting
> > >effect sizes and extracting them from unpromising beginnings.
> > >
> > >It often requires some ingenuity to get the
> > >information you need so have a go and then get
> > >back here with more details if you run into problems
> > >
> > >
> > >Best regards,
> > >
> > >Verena