# The metafor Package

A Meta-Analysis Package for R

### Site Tools

analyses:morris2008

# Differences

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 analyses:morris2008 [2017/03/19 13:09]127.0.0.1 external edit analyses:morris2008 [2021/01/16 09:55]Wolfgang Viechtbauer 2021/11/08 13:17 Wolfgang Viechtbauer 2021/08/27 07:27 Wolfgang Viechtbauer 2021/01/16 09:55 Wolfgang Viechtbauer 2017/03/19 13:09 external edit 2021/11/08 13:17 Wolfgang Viechtbauer 2021/08/27 07:27 Wolfgang Viechtbauer 2021/01/16 09:55 Wolfgang Viechtbauer 2017/03/19 13:09 external edit Next revision Both sides next revision Line 121: Line 121: ==== Alternative Estimates ==== ==== Alternative Estimates ==== - In his article, Morris (2008) discusses two other ways of computing an effect size measure for pretest posttest control group designs. The second approach that pools the two pretest SDs actually can be more efficient under certain conditions. However, that approach assumes that the true pretest SDs are equal for the two groups. That may not be the case. The approach given above does not make that assumption and therefore is more broadly applicable (but may be less efficient). + In his article, Morris (2008) discusses two other ways of computing an effect size measure for pretest posttest control group designs. The second approach that pools the two pretest SDs actually can be more efficient under certain conditions. However, that approach assumes that the true pretest SDs are equal for the two groups. That may not be the case. The approach given above does not make that assumption and therefore is more broadly applicable (but may be slightly less efficient). If you really want to use the approach with pooled pretest SDs, then this can be done as follows: If you really want to use the approach with pooled pretest SDs, then this can be done as follows: Line 139: Line 139: 5 0.44 0.16 5 0.44 0.16 - The ''yi'' values above are the exact same value given in Table 5 (under the $d_{ppc2}$ column) by Morris (2008). Note that the equation used for computing the sampling variances above is slightly different from the one used in the paper, so the values for 'vi' above and the ones given in Table 5 (column $\hat{\sigma}^2(d_{ppc2})$ in Morris, 2008) differ slightly. + The ''yi'' values above are the exact same value given in Table 5 (under the $d_{ppc2}$ column) by Morris (2008). Note that the equation used for computing the sampling variances above is slightly different from the one used in the paper, so the values for ''vi'' above and the ones given in Table 5 (column $\hat{\sigma}^2(d_{ppc2})$ in Morris, 2008) differ slightly. ==== Pretest Posttest Correlations ==== ==== Pretest Posttest Correlations ====