This post is part of an ongoing series formulating a methodology for tracking and understanding the variety of ways in which early Christians received and utilized Scripture.
Echoes are made up of a single significant term, enough to make an inquisitive reader or hearer think about another source, but without the enough evidence to confirm that suspicion and leaving open the possibility of another textual source. These reminiscences are too faint to carry the indicative character of quotations and allusions. Indeed, Paul Foster has questioned whether echoes are anything beyond creative contemporary theological reflections upon ancient texts with no basis in the actual intention or meaning of an author or their text. The possibility of non-literary ideas (or ideas freed from their literary contexts) calls into question any methodology which utilizes echoes as a substantial foundation for making claims about ancient authors, their audiences, or intended meanings.
Several other factors impact the placement of citations along the verbal correspondence spectrum. First, there is a need to look beyond the obvious sources for references, a fact which may serve as a reminder than many ancient texts are no longer extant. In general, while quotations of non-extant sources may be locatable (as in 1 Clement 46:2-5), allusions or echoes to such sources should not be postulated. Second, there is the issue of composite citation, where two sources are conflated and combined in order to create new texts and meanings. In these cases, examination of the composite should occur along verbal line for each component part. Finally, we must recognize that to identify a quotation, allusion, or echo only begins the process of understanding the use of one text in another. Fuller engagement of interpretive concerns are better answered by examining the thematic and authoritative spectrums.
 van den Hoek, 228-229. Emadi, 17. Hays, Echoes, 29. Allison, Intertextual, 19-20. Osburn, 318. Fee, 362. On the limits of defining, locating, and drawing implications from “echoes” see Porter, “Use of OT in NT,” 85-93.  Paul Foster, “Echoes without Resonance: Critiquing Certain Aspects of Recent Scholarly Trends in the Study of the Jewish Scriptures in the New Testament,” JSNT 38.1 (2015): 96.  Ibid., 99, 109.  Ibid., 98-9.  Young, 131. Consider 1 Clement 46.8.