The mean percentage change and the raw mean change are not directly comparable, even after standardization based on the SD of the percentage change or raw change values. So, I would not mix those in the same analysis.
Best, Wolfgang > -----Original Message----- > From: Qiang Yue [mailto:qiangm...@gmail.com] > Sent: Wednesday, May 22, 2013 20:38 > To: Viechtbauer Wolfgang (STAT); r-help > Subject: Re: RE: [R] using metafor for meta-analysis of before-after > studies (escalc, SMCC) > > Dear Dr. Viechtbauer: > > Thank you very much for sparing your precious time to answer my question. > I still want to make sure for the third question below: for studies which > only reported percentage changes (something like: the metabolite > concentration increased by 20%+/-5% after intervention), we can not use > the percentage change to calculate SMCC, but have to get the raw change > first? > > With best wishes. > ________________________________________ > Qiang Yue > > From: Viechtbauer Wolfgang (STAT) > Date: 2013-05-21 10:09 > To: Moon Qiang; r-help > Subject: RE: [R] using metafor for meta-analysis of before-after studies > (escalc, SMCC) > Please see my answers below. > > Best, > Wolfgang > > > -----Original Message----- > > From: r-help-boun...@r-project.org [mailto:r-help-boun...@r-project.org] > > On Behalf Of Moon Qiang > > Sent: Thursday, May 16, 2013 19:12 > > To: r-help > > Subject: [R] using metafor for meta-analysis of before-after studies > > (escalc, SMCC) > > > > Hello. > > > > I am trying to perform meta-analysis on some before-after studies. These > > studies are designed to clarify if there is any significant metabolic > > change before and after an intervention. There is only one group in thes > e > > studies, i.e., no control group. I followed the e-mail communication of > > R-help (https://stat.ethz.ch/pipermail/r-help/2012- > April/308946.html ) and > > the Metafor Manual (version 1.8-0, released 2013-04- > 11, relevant contents > > can be found on pages of 59-61 under 'Outcome Measures for Individual > > Groups '). I made a trial analysis and attached the output here, I wonde > r > > if anyone can look through it and give me some comments. > > I have three questions about the analysis: > > > > 1) Most studies reported the before-and-after raw change as Mean+/- > SD, but > > few of them have reported the values of before-intervention (mean_r and > > sd_r) and the values of after- > intervention (mean_s and sd_s), and none of > > them reported the r value (correlation for the before- and after- > > intervention measurements). Based on the guideline of the Metafor manual > , > > I > > set the raw mean change as m1i (i.e., raw mean change=mean_s=m1i), and s > et > > the standard deviation of raw change as sd1i (i.e., the standard deviati > on > > of raw change =sd_s=sd1i), and set all other arguments including m2i, > > sd2i, > > ri as 0, and then calculated the standardized mean change using change > > score (SMCC). I am not sure if all these settings are correct. > > This is correct. The escalc() function still will compute (m1i- > m2i)/sqrt(sd1i^2 + sd2i^2 - > 2*ri*sd1i*sd2i), but since m2i=sd2i=ri=0, this is equivalent to mean_chan > ge / SD_change, which is what you want. > > Make sure that mean_s is NOT the standard error (SE) of the change scores, > but really the SD. > > > 2) A few studies have specified individual values of m1i, m2i, sd1i, sd2 > i > > , > > but did not report the change score or its sd. So can I set r=0 and use > > these values to calculate SMCC? Since SMCC is not calculated in the same > > way like 1), will this be a problem? > > Yes, this will be a problem, since you now really assume that r=0, which i > s not correct. Maybe you can back- > calculate r from other information (e.g., the p or t value from a t-test - > - see https://stat.ethz.ch/pipermail/r-help/2012- > April/308946.html). Or you could try to get r from the authors (then you c > ould also just directly ask for the change score mean and SD). If that is > not successful, you will have to impute some kind of reasonable value for > r and do a sensitivity analysis in the end. > > > 3) some studies reported the percentage mean changes instead of raw mean > > change (percentage change=(value of after-intervention - value of before > > intervention) / value of before intervention), I think it may not be the > > right way to simply substitute the raw mean change with the percentage > > mean > > changes. Is there any method to deal with this problem? > > Don't know anything off the top of my head. > > > Any comments are welcome. > > > > With best regards. > > ------------------------------ > > Qiang Yue ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
