Detecting heterogeneity

A key element of any meta-analysis is to look for heterogeneity across the studies. This is akin to looking for treatment-by-centre interactions in a multi-centre trial, here we are looking for treatment-by-study interactions. This is done by calculating the statistic  

Alternatively, but less formally, the plot displaying estimates and 95 per cent confidence intervals can be inspected. Homogeneity is confirmed if there is consistency overall across all of the confidence intervals. 

If a lack of homogeneity is seen, the next step is to try to understand why. Clearly, by definition, the studies that have been combined have different protocols and by looking at important features of the studies that reflect those differences, it may be possible to form subgroups of trials that are homogeneous. 

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The paramagnetic probes detected heterogeneity of swelling behavior in smectites that was not obvious from X-ray diffraction data, presumably caused by variation of charge density on different clay layers within the same smectite sample. 

Microscopy. This is a powerful tool for studying visually the distribution of the two phases in the polyblend. One can tell not only the domain size of the dispersed phase but also which polymer forms the dispersed phase from refractive index. A phase contrast light microscope can detect heterogeneity at the 0.2-10 /x level. If the sample can be stained preferentially and sectioned with microtome, then under favorable conditions electron microscopy can show heterogeneity to a very fine scale. In a study of PVC-poly(butadiene-co-acrylonitrile) blend,... 

Source Klomp D, van Laarhoven FL, Scheenen F, Kamm Y, LLeerschap A., Quantitative 191 MR spectroscopy at 3T to detect heterogeneous capecitabine metabolism in human liver, NMR in Biomedicine (2007) 20, 485-492. Copyright (2007) John Wiley Sons. Reprinted with permission.)... 

The dielectric, structural, relaxation time is related to the linear regime. It reflects the evolution of the average permanent dipole moment, linked to the given molecule. The dielectric relaxation time detects heterogeneities indirectly, via changes of the average surrounding of a molecule. It was shown in ref. that dielectric relaxation can be well portrayed, with small distortion only close to Tp, by the MCT critical-like dependence ... 

Charton, E., Davies, C, and McCrae, A. R. (1992). Use of specific polyclonal antibodies to detect heterogeneous lipases from Geotrichum candidum. Biochim. Biophys. Acta 1127, 191-198. 

In contrast to the Laurell rocket technique, this method is not used for quantitative determinations. Rather, it is applied to detect heterogeneity in seemingly homogeneous protein fractions, obtained by gel chromatography or ion exchange chromatography. A set of wells is punched in a... 

Seigneuret, M. Rigaud, J. L. Use of the fluorescent pH probe pyranine to detect heterogeneous directions of proton movement in bacteriorhodopsin reconstituted large liposomes. FEES Lett. 1985, 188,101-106. 

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