**5.3 Weighting factors in synthesis**

By virtue of their emphasis on idiographic knowledge, or the complexities and contradictions of particulars, in some sense qualitative studies resist "summing up"(Light and Pillemer 1984). Then developing a technique to compare the findings of each study, along with determining the methodological comparability, or the similarities and differences among studies is the permanent challenge to meta-analysis. Some researchers argue that a "quality weighting" could be set to weight the studies and then to make the comparison of findings(DiMatteo, Morton et al. 1996). But then the problem of "criteria by qualify" would be introduced inevitably.

The meta-analysis of vulnerability in this study meets the same problem. The factors in category "Geography and Environment" possess the highest total number of mentioned times and the highest percentage of times mentioned. But in some of the documents, the geography of the particular case under investigation is presented simply as background information rather than a contributing factor to hazard vulnerability. Then when determine the relative importance of factors only by counting the frequencies, geography and environmental factors would be the most important, which obviously is a misleading conclusion. On the other hand, if the relative importance is determined by other criteria of weight, such as the background of authors, the disciplines, the geographic affiliations, it would plunge in the bias of quality or sampling again.

Also the weighting of factors is related directly to the outputs of meta-analysis. Additionally, how to weight the difference and the similarity between studies is a complex problem which depends on the aims of meta-analysis, methods employed, criteria of selection and even the expectation of outputs.

Meta-analysis is a systematic framework that could be applied in the synthesis and comparison of accumulated studies, no matter literatures or field data. Unlike quantitative meta-analysis, in the qualitative research field the methods employed in meta-analysis is various according to different studies. Currently the main methods used in qualitative metaanalysis are still vote-counting or similar methods (Geist and Lambin 2001; Kevale 2001; Misselhorn 2005). But this is an inexact approach to integrating research, because it depends on the sample size very much. In fact, the wide variety of presentation ways, the artificial lines drawn in research reports among methods, results, discussions and findings are all challenges to the meta-analysis methods. Therefore, progressed approaches are expected in qualitative meta-analysis to match the progressed research framework.

From the view of applied fields, meta-analysis approach has been used for a long time mainly in the field of experimental medicine, clinical pharmacology, and behavioral sciences. Also it has been used in quasi or non-experimental contexts of economic research and some social science studies (Travisi, Florax et al. 2004). For example, a meta-analysis funded by New Zealand government provided insights into the lessons learnt from 10 very different community action projects funded by a range of government agencies in New Zealand, and The analysis has been used to inform a framework for community action projects, which identifies key developmental practices that will strengthen similar projects (Greenaway, Milne et al. 2004). It has begun to be used in some fields linked to environmental problems or climate change. In the work of Gaarder (2002), a regression analysis is undertaken using the large sample of air pollution mortality studies to date, from both developing and developed countries, to further the understanding of the relationship between suspended particles and mortality(Gaarder 2002).
