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[R-meta] Data Extraction
2 messages · Tobias Saueressig, Wolfgang Viechtbauer
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 at r-project.org] On Behalf Of Tobias Saueressig Sent: Monday, 17 August, 2020 12:03 To: r-sig-meta-analysis at 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