One of the most recent citations (Shull et al. 2002) is freely downloadable:

https://www.computer.org/csdl/proceedings-article/metrics/20...

Its own citations may not be satisfying, but I find it nevertheless interesting. Here's the summary of the eWorkshop discussion of "Effort to find and fix":

> A 100:1 increase in effort from early phases to post-delivery was a usable heuristic for severe defects, but for non-severe defects the effort increase was not nearly as large. However, this heuristic is appropriate only for certain development models with a clearly defined release point; research has not yet targeted new paradigms such as extreme programming (XP), which has no meaningful distinction between "early" and "late" development phases.