Field- and time-normalization of data with many zeros: An empirical analysis using citation and Twitter data
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Thelwall (2017a, 2017b) proposed a new family of field- and time-normalized indicators, which is intended for sparse data. These indicators are based on units of analysis (e.g., institutions) rather than on the paper level. They compare the proportion of mentioned papers (e.g., on Twitter) of a unit with the proportion of mentioned papers in the corresponding fields and publication years (the expected values). We propose a new indicator (Mantel-Haenszel quotient, MHq) for the indicator family. The MHq goes back to the MH analysis. This analysis is an established method, which can be used to pool the data from several 2x2 cross tables based on different subgroups. We investigate (using citations and assessments by peers, i.e., F1000Prime recommendations) whether the indicator family (including the MHq) can distinguish between quality levels defined by the assessments of peers. Thus, we test the convergent validity. We find that the MHq is able to distinguish between quality levels (in most cases) while other indicators of the family are not. Since our study approves the MHq as a convergent valid indicator, we apply the MHq to four different Twitter groups as defined by the company Altmetric (e.g., science communicators). Our results show that there is a weak relationship between all four Twitter groups and scientific quality, much weaker than between citations and scientific quality. Therefore, our results discourage the use of Twitter counts in research evaluation.
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