The surface brightness distributions for all shell SNRs in Green's catalog (solid line), the 20 shell SNRs within 3 kpc of the Sun (dashed line), and the sample of 36 shell SNRs (excluding Cas A) with known distances (dot‐dashed line). The sample with known distances is biased toward brighter remnants. The distributions are fit to Gaussians with a lower cutoff at Σ_c = 5 × 10⁻²³ W m⁻² Hz⁻¹ sr⁻¹. The distribution for shell SNRs within 3 kpc of the Sun is used to determine scale factors to compensate for observational selection effects (see § 4).