How can I conduct an Egger's test using SPSS17? For each study included in the meta-analysis I know effect size and sample size of patients and controls groups.


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In order to conduct Egger's regression test you will also need the standard errors ($SE_i$) of your effect sizes ($ES_i$). Then generate the so called standard normal deviate (SND) which is defined as effect size divided by its standard error ($ES_i / SE_i$). Next, generate the precision which is $\frac{1}{SE_i}$. The regression model is: $SND = a + b \cdot precision$ (I know the error term is missing but let's keep it simple). Finally, estimate this regression model (unweighted) in SPSS/PASW (see Egger et al 1997: "Methods: Measures of funnel plot asymmetry").

The logic of Egger's regression test in explained in another CrossValidated thread: "Egger’s linear regression method intercept in meta analysis".

  • $\begingroup$ @Andrej @Bernd Thank you for your time. I see that Cochrane handbook, Egger's paper(PMID9310563) and MIX2.0 software use different funnelplots, but they all agree that effect size on the horizontal axon and a meassure of sample size on the vertical axon. I don't have means and standard deviations of the groups (patients, controls) of each study. I have only the effect size, and the sample size n1,n2 of patients and controls. So I cannot calculate SE of the effect sizes to use them in the linear regression. Could I use only a funnel plot with samplesize on vertical and effectsize on horizontal? $\endgroup$ Feb 20, 2011 at 23:14
  • $\begingroup$ @Staty Despair: Please be more specific about what type of effect size you have (odds ratio, risk ratio...). Do you know the Practical Meta-Analysis Effect Size Calculator? This tool might help you to get the standard errors. $\endgroup$ Feb 20, 2011 at 23:31
  • $\begingroup$ @Bernd @Andrej Thanks again. I don't know what type is the effect size (SMD, OR, RR etc). I only know that can take values from -1 to +1, to show the size and the direction of the difference. The variable compared between patient and control groups is a typical scale variable, but no means and standard deviations available from the included studies. The effect size is called SDM value (www.sdmproject.com). I know (automatically calculated) the effect size from each included study, but not the formula used for the calculation. I also now sample sizes n1, n2 of patients,controls in each study. $\endgroup$ Feb 21, 2011 at 0:07
  • $\begingroup$ @Bernd this website includes probably the most complete and well organized collection of online calculators, ...but still not helpful in my case :( $\endgroup$ Feb 21, 2011 at 0:18
  • $\begingroup$ @Staty Despair: I am sorry but I do not have any experience with this type of analysis. You might want to check this paper: Voxel-wise meta-analysis of grey matter changes in obsessive–compulsive disorder. $\endgroup$ Feb 21, 2011 at 0:24

I don't use PASW anymore, but implementation of the Egger's test for asymmetry is quite simple. First please look at the Egger's paper where he propose "theory" behind the test.

Basically you have two variables: (i) normalized effect estimate (your estimate divided by its standard error), and (ii) precision (reciprocal of the standard error of the estimate). Then you should conduct simple linear regression and test for intercept $\beta_0 = 0$.

  • $\begingroup$ I see... Egger's test of the null hypothesis that intercept b=0 (or tests the null hypothesis that there is no funnel plot asymmetry). In this case the regression line will run through the origin. If the intercept b deviates from zero, the intercept b provides a meassure of asymmetry. The larger the interceptor's deviation from zero point, the larger the asymmetry. The two-sided p-value should be reported. (Synopsis from the book Publication bias in meta-analysis: prevention, assessment and adjustments. Po avtorjih A. J. Sutton). Thanks. $\endgroup$ Feb 23, 2011 at 2:34

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