Empirical Musicology Review
The parent article looks at pitch stability in an archive of folksongs recorded over several decades. Some evidence for pitch stability was found. Here, I consider some additional aspects of the archive that could be examined, offer some extensions to relevant laboratory studies, and consider some inherent strengths and limitations of the naturalistic, archival approach.
Halpern, Andrea R.. "Absolute Pitch in Naturalistic Singing: A Commentary On Olthof et al.." Empirical Musicology Review (2015) : 175-177.