[R] Meta-analysis of prevalence at the country level with mgcv/gamm4

[Michael Dewey]

At 13:17 10/04/2014, Julien Riou wrote:
> >
> > Message: 11
> > Date: Wed, 09 Apr 2014 18:39:30 +0100
> > From: Michael Dewey <info at aghmed.fsnet.co.uk>
> > To: Julien Riou <julien.riou.k at gmail.com>, r-help at r-project.org
> > Subject: Re: [R] Meta-analysis of prevalence at the country level with
> >         mgcv/gamm4
> > Message-ID: <Zen-1WXwTL-0005q4-0V at smarthost01a.mail.zen.net.uk>
> > Content-Type: text/plain; charset="us-ascii"; format=flowed
> >
>
>
>Hi Michael,
>
>Thank you for taking the time to help me.
>
>  So the first UK study with a median age of 25 is going to be used to
> > estimate prevalence over a range of ages? You are going to have to
> > make some very strong assumptions here which I personally would not
> > want to make.
> >
>
>I'm a little confused by this. In my understanding, the mixed-effects model
>does not do that. The slope of the relation between age and prevalence will
>be estimated from the full pool of studies, and the country-level random
>intercept will be estimated from all studies in the country. So the
>assumption here is that the relation between age and incidence is the same
>in every country, which is quite reasonable. Of course, there will be more
>uncertainty with the estimation of the random intercept if there is few
>studies in a country, or if there is a strong inter-study variance in a
>country. This will influence the confidence interval of the random
>intercept, and so the CI of the predicted prevalence for this country.

Your studies are ecological. You are estimating the relationship 
between prevalence and being in a study of median age X which is not 
necessarily the same as the relationship between prevalence and being 
a person of age X.



>Is there any possibility that in the real dataset you can fit your
> > model to those studies which do provide age-specific prevalences and
> > then use that to impute?
> >
> > You do not say when these studies were published but I would ask the
> > authors of the primary studies if they can make the information
> > available to you. You may have already done that of course. I referee
> > quite a few papers on systematic reviews and my impression is that
> > some authors are amenable to doing the work for you. You mileage may
> > vary of course.
> >
>
>Yes, it would be easier to have prevalence for age subgroups of studies,
>but we did not have access to that information for most studies even after
>contacting the authors.
>
>
> > >*Standard random-effect meta-analysis* with package meta.
> > >
> > >I used metaprop() to get a first estimate of the prevalence in each
> > country
> > >without taking age into account, and to obtain weights. As expected,
> > >heterogeneity was very high, so I used weights from the random-effects
> > >model.
> >
> > Which will be nearly equal and so hardly worth using in my opinion
> > but again your mileage may vary.
> >
>
>The weights from the random-effects method were actually far from equals,
>as sample size was highly variable between studies. With the RE method,
>small studies have much more impact.
>
>
> > I am afraid that is the way with systematic reviews, you can only
> > synthesise what you find, not what you would like to have found.
> > Anyone who has done a review will sympathise with you, not that that
> > is any consolation.
> >
>
>I'm not sure I'm following your point. My objective is to synthesise the
>included studies, while taking the age factor into account, since it is
>strongly linked to prevalence and very heterogeneous. The alternative is to
>only include studies with low median age, but I would lose a lot of
>information.
>
>Thank you again,
>Julien
>
> >
> >
>
>         [[alternative HTML version deleted]]

Michael Dewey
info at aghmed.fsnet.co.uk
http://www.aghmed.fsnet.co.uk/home.html