  Through a very careful analysis Kolopanis and collaborators identified a
negative power spectrum (PS) systematic. The 21 cm cosmology community has
assumed that any observational systematics would add power, as negative PS are
non-physical. In addition to the mystery of their origin, negative PS
systematics raise the spectre of artificially lowering upper limits on the 21
cm PS. It appears that the source of the negative PS systematics is a subtle
interaction between choices in how the PS estimate is calculated and
baseline-dependent systematic power. In this paper we present a statistical
model of baseline dependent systematics to explore how negative PS systematics
can appear and their statistical characteristics. This leads us to
recommendations on when and how to consider negative PS systematics when
reporting observational 21 cm cosmology upper limits.
