[R-meta] Calculating SMD from posttest only - is this possible?
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