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The “weight” of a trial in meta-analysis

You’ve probably all seen a forest plot in a meta-analysis – it looks a bit like this:

Forest plot

I’m not going to explain a forest plot. This post is just about “weight”.

To calculate the contribution that each study should have on the position of the big diamond blob, each study is “weighted”.

How is it weighted you ask… well the weight of each study is calculated from the standard error of the relative risk in that study (on a log scale).

Weight= \frac{1}{standard\: error^{2}}

Larger trials usually have smaller standard errors, so they will usually carry more weight in the meta-analysis. This is not necessarily the case though – the number of events also influences the standard error (a large trial may still have a large standard error if the number of events was small).

To work out where the diamond blob should go, you simply smash out the below equation (basically a weighted mean):

Combined\:effect\: size= \frac{sum\, of\, all\left ( effect\: size\times weight \right )}{sum\,of\,all\,the\,weights}
The weights are often displayed as a percentage in the forest plot – this percentage is not used in the equations above.

The numerical equation for this weighted mean (combined effect size) is below if you really want it…

Show me the equation!

\bar T\cdot = \frac{\sum w_{i}T_{i}}{\sum w_{i}}

\bar T\cdot = combined effect size (the weighted mean)

Σ= sum of…

Wi = individual study weight

Ti = individual study effect size

 

  • A concice guide to clinical trials by Allan Hackshaw
  • Handbook of parametric and nonparametric statistical procedures by David Sheskin
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ECGs

ECGs!

This is more of a website experiment that it is a useful medical post.

I just wanted to see if I could get an ECG trace behind the title.

Sorry.

Of course I will be, in due course, creating a distilled library of useful ICM-related ECGs with handy tips borrowed from the #FOAM word’s best humans.

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ICM… distilled

The main aim of this site is to provide me with a place to store the things I learn in intensive care medicine. If a happy offshoot of this is that others can learn a bit too, then so much the better.
This site does not promise to be comprehensive. Indeed a lot of it will be heavily curated and condensed. Topic pages should, however, be sufficient for that ward-round quizzing, or that viva, or that 2am mind blank.

Links to further reading, where more in-depth knowledge can be garnered, will be provided where appropriate.

This is intensive care medicine… distilled.

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Well hello there…

Hello. It’s unlikely that anybody will see this for some time but the blog had to start somewhere.

This site hopes to be a source of curated distilled ICM knowledge as a source of revision and continuing professional development.

I’ll get started on creating some content…