I am computing effect sizes from various studies for a meta-analysis. Most of those studies follow a typical pretest-posttest design (repeated measures), thus I compute the effect size as a "Standardized Mean Change".
I am using the
metafor package in
R to compute these effect sizes, but it asks for the correlation coefficient (𝑟,
metafor) between pre and posttest scores.
My formula is something like this:
escalc(data = df, measure = "SMCR", m1i = mean_pre, m2i = mean_post, sd1i = sd_pre, sd2i = sd_post, ni = n_subjects, append = T) # Error in escalc.default(data = df, measure = "SMCR"... : # Cannot compute outcomes. Check that all of the required # information is specified via the appropriate arguments.
→ It requires
ri (correlation coefficient).
The problem is that no study ever reports correlation coefficients for a pretest-posttest design. And I don't think I can compute it from the summary statistics (M, SD). How should I proceed?