[R-meta] Q-Statistic for Between-Subgroup Heterogeneity

[Ty Beal]

Dear Wolfgang,

Thanks for the tips. I am still getting used to the mailing list. I read through the thread and still have a couple questions, which will require me to explain my analysis in more detail.

I am conducting a meta-analysis of prevalence for four dietary questions based on frequency of consumption. For example, the first question is coded so that I am calculating the prevalence that individuals consume fruit less than once per day. My data include 72 countries with nationally representative data that are weighted according to a complex survey design. This requires me to use the survey package to calculate the prevalence and SE/CIs for each of the 72 countries.

I have done the meta-analysis for all the studies together and calculated an I^2 statistic. Now I am making a forest plot of subgroups (5 regions--and later also by sex). I would like to present some measure of between subgroup heterogeneity, but I am not sure the best approach to take. I had been using the following code to obtain a Q-statistic:

fruit_res_r <- rma(yi, vi, method="DL", mods = ~ factor(region), data=fruit)

But after reading the post that Wolfgang added, I realized that I am comparing the first region as the reference level. When I use factor(region) - 1, it produces much higher Q-statistics and lower P-values. I just want to be sure this is the best approach for this context.

Best,
Ty

On 7/24/18, 5:58 PM, "Viechtbauer, Wolfgang (SP)" <wolfgang.viechtbauer using maastrichtuniversity.nl> wrote:

    Dear Ty,

    Please always CC the mailing list.

    And please re-read the posts under the link I posted (i.e., the entire thread). In particular, this one is most relevant:

    https://stat.ethz.ch/pipermail/r-sig-meta-analysis/2017-December/000431.html

    It shows how to fit a model with different tau^2 values per subgroup using metafor. See also:

    http://www.metafor-project.org/doku.php/tips:comp_two_independent_estimates

    Best,
    Wolfgang

    -----Original Message-----
    From: Ty Beal [mailto:tbeal using gainhealth.org]
    Sent: Tuesday, 24 July, 2018 17:18
    To: Viechtbauer, Wolfgang (SP)
    Subject: RE: [R-meta] Q-Statistic for Between-Subgroup Heterogeneity

    Dear Wolfgang,

    If you do end up updating metafor to allow different between-subgroup heterogeneity, please let me know (if you remember). I may want to re-run the analysis to see how big of a difference it makes in uncertainty estimates.

    Best,
    Ty

    -----Original Message-----
    From: R-sig-meta-analysis <r-sig-meta-analysis-bounces using r-project.org> On Behalf Of Viechtbauer, Wolfgang (SP)
    Sent: Monday, July 16, 2018 8:22 AM
    To: r-sig-meta-analysis using r-project.org
    Subject: Re: [R-meta] Q-Statistic for Between-Subgroup Heterogeneity

    Good point. This was discussed before (for metagen(), but it's the same issue):

    https://stat.ethz.ch/pipermail/r-sig-meta-analysis/2017-December/000430.html

    Best,
    Wolfgang

    -----Original Message-----
    From: R-sig-meta-analysis [mailto:r-sig-meta-analysis-bounces using r-project.org] On Behalf Of Guido Schwarzer
    Sent: Monday, 16 July, 2018 13:13
    To: r-sig-meta-analysis using r-project.org
    Subject: Re: [R-meta] Q-Statistic for Between-Subgroup Heterogeneity

    Am 13.07.18 um 18:31 schrieb Viechtbauer, Wolfgang (SP):

    > [...]
    > In the output, you will find:
    >
    > Test of Moderators (coefficients 2:3):
    > QM(df = 2) = 1.7675, p-val = 0.4132
    >
    > This is the test for between-subgroup heterogeneity.

    ... assuming a common between-study variance tau2 across subgroups. The default in metaprop() of meta is to allow different between-study variances (analogous to RevMan 5, the software for Cochrane Reviews).
    However, this can be changed using argument 'tau.common = TRUE' in metaprop().

    As stated by Ty, metaprop() does not allow to specify the weights at the moment. I put this on my TODO list.

    Best wishes,
    Guido


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