[R-meta] Pre-test Post-test Control design Different N

Michael Dewey ||@t@ @end|ng |rom dewey@myzen@co@uk
Wed Jan 13 15:28:41 CET 2021

Dear Marianne

Perhaps you could clarify something for us. You state that you have a 
pre-post design and you have used methods for paired data. You then go 
on to describe situations in which the number of units before and after 
is not the same. Perhaps it is just me but I do not understand how you 
can then have pairing.


On 13/01/2021 08:04, Marianne DEBUE wrote:
> Hi,
> I'm conducting a meta-analysis in ecology on a Pre-test Post-test Control design.
> I'd like to use Morris "dppc2" formula (in Estimating effect sizes from pretest-posttest-control group designs , [ https://doi.org/10.1177%2F1094428106291059 | https://doi.org/10.1177/1094428106291059 ] ) in order to take into account the non-independency of the pre-test post-test design.
> This formula applies for paired-observations and depends on the Pre-test Control Mean, Post-test Control Mean, Pre-test Treatment Mean, Post-test Treatment Mean, Pre-Test Control SD, Pre-test Treatment SD, Treatment Sample size nT and Control Sample size nC.
> Some studies have the same pre-test and post-test nT (and nC) because they always sample the same plots. However, so me studies have a different Pre-Test nT and Post-Test nT (and/or pre-Test nC and Post-Test nC), either because of the experimental design of the author of the study (for example 30 Before samples and 60 After samples), or because we have gathered the Before data if they were given for several years using the Cochrane combined group formula ( [ https://handbook-5-1.cochrane.org/chapter_7/table_7_7_a_formulae_for_combining_groups.htm | https://handbook-5-1.cochrane.org/chapter_7/table_7_7_a_formulae_for_combining_groups.htm ] ) (for example, 30 samples taken 2 years before the intervention, 30 samples taken one year before the intervention, and 30 samples taken one year after the intervention, giving 60 samples taken before the intervention and 30 taken after).
> Do you know if it is possible to adapt the formula to take into account a possible difference in nT or nC between the Before and After?
> Best regards,
> Marianne
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