Message-ID: <08882a96272a4f74975f93d3dbc853f8@UM-MAIL3214.unimaas.nl>
Date: 2019-10-07T20:13:32Z
From: Wolfgang Viechtbauer
Subject: [R-meta] Calculating SMD from posttest only - is this possible?
In-Reply-To: <e04b9ac1-7be4-d1be-18ca-116080bbed9e@imbi.uni-freiburg.de>
That is correct when using standardized mean change values using change score standardization -- and indeed, that's not something that should done. However, the standardized mean change using raw score standardization also standardizes based on the variability at a single time point and hence is in principle comparable to the standardized mean difference at the posttest (leaving aside other issues, such as time effects). I would still be very careful when doing this and I very much like Michael's suggestion to examine this via a moderator analysis.
Best,
Wolfgang
-----Original Message-----
From: R-sig-meta-analysis [mailto:r-sig-meta-analysis-bounces at r-project.org] On Behalf Of Guido Schwarzer
Sent: Monday, 07 October, 2019 14:16
To: P. Roberto Bakker; r-sig-meta-analysis at r-project.org
Subject: Re: [R-meta] Calculating SMD from posttest only - is this possible?
Hi Roberto,
Just to add to the responses by Wolfgang and James.
The Cochrane Handbook warns against combining SMDs based on change
scores (pre-post comparisons) and post-test comparisons (see
https://training.cochrane.org/handbook/current/chapter-10#section-10-5-2).
The reason is that the standard deviation used in the denominator of the
SMD is different for change scores and post-tests.
Best wishes, Guido
--
Dr. Guido Schwarzer
Institute of Medical Biometry and Statistics,
Faculty of Medicine and Medical Center - University of Freiburg
Postal address: Stefan-Meier-Str. 26, D-79104 Freiburg
Phone: +49/761/203-6668
Mail: sc at imbi.uni-freiburg.de
Homepage: http://www.imbi.uni-freiburg.de
ORCID iD: https://orcid.org/0000-0001-6214-9087
R-book: https://www.springer.com/gp/book/9783319214153