When combining continuous data using meta-anlyses most often we use mean, standard deviation and number of sample in each group. Is it possible to combine data reported as median and interquartile range? Is there anyone who has experience in this regard?
Median and IQR are used when there is not a normal distribution of data, and that is going to be a problem in the metanalysis.
There are 3 options:
1. Ask for the mean and std dev to the authors.
2. Exclude those papers. One of the inclusion criteria of a paper in a metanalysis must be that you can have all the data that you need to do the analysis.
3. Calcuate the SD and the mean. This is not the best way!! If you assume hat those data have a normal distribution you could think that the mean is going to be similar to the median. One way to calculate the SD, assuming normal distribution, is to divide the IQR by 1,35, that is approximately the equivalence in SD of the IQR.
For example, if you IQR is 4,0 - 6,0, and the median is 5, then:
2,0 (IQR) divided by 1,35 is = 1,48
So the "new" mean and SD will be: 5,0 (SD 1,48)
This is not the best way, but if you can't do the first two, this an optional way.
Try the book "Statistical Meta Analysis with applications" - Joachim Hartung, Guido Knapp, Bimal K. Sinha
The attached publication just came out in BMC Medical Research Methodology. The authors also provide an Excel spreadsheet for calculating the estimates,
Xian W. et al. "Estimating the sample mean and standard deviation from the sample size, median, range and/or interquartile range." Hope you find it useful.
That is an excellent contribution. Thank you Johnny, Subrata and Branko. Your discussions and the articles you provided me were very nice.
Thank you Garumma for posting this question, I experienced the same thing.
What is the cut off sample size ( defined as small and defined as large) to use the Cochrane Book recommended formula for estimated SD= q3 − q1/1.35. (Higgins JPT, Green S: Cochrane Handbook for Systematic Reviews of Interventions. Wiley Online Library; 2008.)?
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