The exploration of possible sources of heterogeneity between studies should be part of any Cochrane review, and is discussed in detail in Chapter 9 (Section 9.6). Non-randomized studies may be expected to be more heterogeneous than randomized trials, given the extra sources of methodological diversity and bias. The simplest way to show the variation in results of studies is by drawing a forest plot (see Chapter 11, Section 11.3.2).
It may be of value to undertake meta-regression analyses to identify important determinants of heterogeneity, even in reviews where studies are considered too heterogeneous to pool. Such analyses may help to identify methodological features which systematically relate to observed intervention effects, and help to identify the subgroups of studies most likely to yield valid estimates of intervention effects.