Message-ID: <80acb766352946fcb55145b287dfea09@UM-MAIL3214.unimaas.nl>
Date: 2020-08-18T06:50:00Z
From: Wolfgang Viechtbauer
Subject: [R-meta] Data Extraction
In-Reply-To: <trinity-c006fd21-a7ac-42bd-b2b3-ca4384ddef29-1597729179616@3c-app-gmx-bap22>
Please always cc the mailing list.
You can include the study if you know (or can guestimate) the pre-post correlation.
SD of the change scores = SD * sqrt(2*(1-r)),
where SD is the pre or post treatment SD (this assumes that the SD is the same before and after the treatment). So if you know r, you can easily recover the SD and standardize in the usual manner.
Best,
Wolfgang
>-----Original Message-----
>From: Tobias Saueressig [mailto:t.saueressig at gmx.de]
>Sent: Tuesday, 18 August, 2020 7:40
>To: Viechtbauer, Wolfgang (SP)
>Subject: Aw: RE: [R-meta] Data Extraction
>
>Dear Wolfgang,
>
>you are my hero. Thank you.
>
>I am a bit sad that I cannot include the study because it is based on change
>scores and I have used postintervention only.
>
>Gesendet:?Montag, 17. August 2020 um 22:33 Uhr
>Von:?"Viechtbauer, Wolfgang (SP)"
><wolfgang.viechtbauer at maastrichtuniversity.nl>
>An:?"Tobias Saueressig" <t.saueressig at gmx.de>, "r-sig-meta-analysis at r-
>project.org" <r-sig-meta-analysis at r-project.org>
>Betreff:?RE: [R-meta] Data Extraction
>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