[R] using metafor for meta-analysis of before-after studies (escalc, SMCC)
qiangmoon at gmail.com
Thu May 16 19:11:42 CEST 2013
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 these
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 wonder
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 set
the standard deviation of raw change as sd1i (i.e., the standard deviation
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.
2) A few studies have specified individual values of m1i, m2i, sd1i, sd2i ,
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?
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?
Any comments are welcome.
With best regards.
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