  21 cm power spectrum observations have the potential to revolutionize our
understanding of the Epoch of Reionization and Dark Energy, but require
extraordinarily precise data analysis methods to separate the cosmological
signal from the astrophysical and instrumental contaminants. This analysis
challenge has led to a diversity of proposed analyses, including delay spectra,
imaging power spectra, m-mode analysis, and numerous others. This diversity of
approach is a strength, but has also led to confusion within the community
about whether insights gleaned by one group are applicable to teams working in
different analysis frameworks. In this paper we show that all existing analysis
proposals can be classified into two distinct families based on whether they
estimate the power spectrum of the measured or reconstructed sky. This subtle
difference in the statistical question posed largely determines the
susceptibility of the analyses to foreground emission and calibration errors,
and ultimately the science different analyses can pursue. In this paper we
detail the origin of the two analysis families, categorize the analyses being
actively developed, and explore their relative sensitivities to foreground
contamination and calibration errors.
