Eta is another dispersion-based measure recently adapted by Du et al. (2021) for comparative analysis of two corpora. It is based on the Deviation of Proportions (DP) suggested by Gries (2008). DP is obtained by comparing the relative size of each text in a corpus to the relative frequency of a target word in each text and summing up all differences. Like Zeta, Eta therefore also measures the degree of even distribution of a feature, but it does so in a different way, namely, by comparing the DPs of words in two corpora to identify which words have a stronger dispersion in the target corpus. 

References

Gries, Stefan Th, ‘What Do (Most of) Our Dispersion Measures Measure (Most)? Dispersion?’, 2021 <https://doi.org/10.1075/jsls.21029.gri>
Gries, Stefan, ‘A New Approach to (Key) Keywords Analysis: Using Frequency, and Now Also Dispersion’, Research in Corpus Linguistics, 9 (2021), 1–33 <https://doi.org/10.32714/ricl.09.02.02>
Du, Keli, Julia Dudar, Cora Rok, and Christof Schöch, ‘Zeta & Eta: An Exploration and Evaluation of Two Dispersion-Based Measures of Distinctiveness’, Proceedings Http://Ceur-Ws. Org ISSN, 1613 (2021), 0073
Gries, Stefan Th, ‘Analyzing Dispersion’, Practical Handbook of Corpus Linguistics. New York, NY: Springer, 2019
Gries, Stefan Th., ‘The Most Under-Used Statistical Method in Corpus Linguistics: Multi-Level (and Mixed-Effects) Models’, Corpora, 10.1 (2015), 95–125 <https://doi.org/10.3366/cor.2015.0068>
Lijffijt, Jefrey, and Stefan Th. Gries, ‘Correction to Stefan Th. Gries’ “Dispersions and Adjusted Frequencies in Corpora”, International Journal of Corpus Linguistics’, International Journal of Corpus Linguistics, 17.1 (2012), 147–49 <https://doi.org/10.1075/ijcl.17.1.08lij>
Gries, Stefan Th., ‘Useful Statistics for Corpus Linguistics’, in A Mosaic of Corpus Linguistics: Selected Approaches, ed. by Aquilino Sánchez and Moisés Almela (Frankfurt am Main: Peter Lang, 2010), pp. 269–91
Gries, Stefan Th., ‘Dispersions and Adjusted Frequencies in Corpora: Further Explorations’, Language and Computers, 2009, 197–212
Gries, Stefan Th., ‘Dispersions and Adjusted Frequencies in Corpora’, 2008 <https://doi.org/10.1075/ijcl.13.4.02gri>
Gries, Stefan Th., Statistik für Sprachwissenschaftler (Vandenhoeck & Ruprecht, 2008)