[R-meta] Data Extraction

[Viechtbauer, Wolfgang (SP)]

Dear Tobias,

The difference between the upper and lower CI bounds divided by the appropriate critical t-value should be *twice* the SE. So:

SE <- (9.3 - -5.7) / (2*1.99) 

should be the SE of the difference in the mean change between the two groups. Dividing this by sqrt(1/n1 + 1/n2) should give you the SD of the change scores (assuming homoscedasticity of the change variances in the two groups), so:

SD <- SE / sqrt(1/30 + 1/36)

Hence:

1.8/SD 

yields approximately 0.12. Sure you can apply the bias correction, but this is hardly relevant here.

More importantly, this d-value standardizes the difference between the two groups based on the (pooled) SD of the change scores. This is not comparable to d-values that use the SD of a single time point for the standardization.

Best,
Wolfgang

>-----Original Message-----
>From: R-sig-meta-analysis [mailto:r-sig-meta-analysis-bounces using r-project.org]
>On Behalf Of Tobias Saueressig
>Sent: Monday, 17 August, 2020 12:03
>To: r-sig-meta-analysis using r-project.org
>Subject: [R-meta] Data Extraction
>
>Hello,
>
>I have a problem to extract data from one study.  I want to calculate hedges
>g.
>
>I have the following information (2 groups):
>
>Between-group change score from baseline to follow-up (9 months) is 1.8(-
>5.7,9.3) (mean and 95%CI) and p-value 0.64
>
>Sample size is n1 = 30; n2 = 36 (two group RCT).
>
>Link: https://linkinghub.elsevier.com/retrieve/pii/S1063458420309882 (TABLE
>2 HOOS ADL)
>
>I would do it the following way:
>
>Calculate a SD for both group from the CI. _> SE = (Upper limit-lower
>limit)/t-statistic (~2) => SD = SE/squ(1/n1 + 1/n2)
>
>Then g = d = MD/SD
>
>g* = (1-3/(4*(n1+n2)-9) * g
>
>e.g.
>
>SE = (9.3+5.7)/1.99 = 7.54 => SD = 7.54/squ(1/30+1/36) = 7.54/0.25 = 30.16
>
>g = 1.8/30.16 = 0.06
>
>g* = 0.06 * (1-3/255) = 0.06 * 0.99 = 0.0594
>
>Is that correct?
>
>Best regards
>
>Tobias