[R] Metafor: How to integrate effectsizes?

Michael Dewey info at aghmed.fsnet.co.uk
Mon May 26 10:56:21 CEST 2014


At 13:10 22/05/2014, Verena Weinbir wrote:
>Hello,
>
>thank you very much for your replies. I am 
>almost done :-) but theres one study left, where 
>I only have sample size (not group size),  mean 
>values and standarddeviations. Is there a way to 
>compute cohens d from this data?

You could assume that the split was equal. If you 
are worried you could, as a sensitivity analysis, 
assume a plausible range of splits and see if 
that affects the outcome. If these are 
experiments or trials it is unlikely people would 
have arranged anything too asymmetrical.


>I thought it was correct to use measure="SMDH" 
>in the escalc () function to compute cohens d? 
>In your illustration, Wolfgang, you use SMD as measure - now I am confused ;-)

People usually use SMD I think, the other relaxes 
the assumption about homogeneity of variances.


>Thank you very much in advance!
>
>Best,
>Verena
>
>
>On Tue, May 6, 2014 at 7:14 PM, Michael Dewey 
><<mailto:info at aghmed.fsnet.co.uk>info at aghmed.fsnet.co.uk> wrote:
>At 14:23 06/05/2014, Viechtbauer Wolfgang (STAT) wrote:
>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).
>
>
>Verena,
>What Wolfgang says is true of course but if you 
>have _both_ the t value and the p value you can 
>backcalculate the number of degrees of freedom 
>and then if you are willing to assume equal arms you have the sample sizes.
>
>finddf <- function(t, pval) {
>Â  Â helper <- function(df) {res <- pval - pt(t, df, lower.tail = FALSE); res}
>Â  Â res <- uniroot(helper, interval = c(5, 10000))
>Â  Â res
>}
>
>If you call finddf with the value of t and the 
>_one-sided_ p-value (divide by 2 if two-sided) 
>it should give you a return value which, if you 
>look at the element of the list called root is 
>its estimate of the degrees of freedom. If you 
>get errors from uniroot the interval supplied in 
>the call may need to be widened.
>
>I would suggest that when you have your final 
>dataset it would be a really good idea to do 
>some model checks using plot.influence to see 
>whether the studies for which you have imputed 
>values are fundamentally different for some 
>reason. This will also check your calculations as a bonus.
>
>
>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); 
> <mailto:r-help at r-project.org>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 <<mailto:info at aghmed.fsnet.co.uk>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>http://www.metafor-project.org/doku.php/tips:assembling_data_smd
> >
> > Verena might also like to look at the 
> <http://compute.es>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) 388-4170Â | <http://www.wvbauer.com>http://www.wvbauer.com
>
> >
> > > -----Original Message-----
> > > From: 
> <mailto:r-help-bounces at r-project.org>r-help-bounces at r-project.org 
> [mailto:<mailto:r-help-bounces at r->r-help-bounces at r-
> > <http://project.org>project.org]
> > > On Behalf Of Michael Dewey
> > > Sent: Friday, April 25, 2014 16:23
> > > To: Verena Weinbir
> > > Cc: <mailto:r-help at r-project.org>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><mailto:in 
> fo 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 verry 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 H and Hedges, L V},
> > > >Â  Â title = {A handbook ok of research synthesis},
> > > >Â  Â year = {1994},
> > > > >Â  Â publisher = {Russellell Sage},
> > > >Â  Â address = {New York}rk},
> > > >Â  Â keywords = {meta-anaanalysis}
>
> > > >}
> > > >(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
>
>
>Michael Dewey
><mailto:info at aghmed.fsnet.co.uk>info at aghmed.fsnet.co.uk
>http://www.aghmed.fsnet.co.uk/home.html
>

Michael Dewey
info at aghmed.fsnet.co.uk
http://www.aghmed.fsnet.co.uk/home.html



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