Comparison of the 68% and 95% probability regions for the amplitude and slope of a power-law fit to the observed GWB signal (green contours) and predicted for purely GW-driven SMBHB populations with circular orbits (blue contours; NG15smbh). The black dashed lines represent a 2D Gaussian fit of the blue contours. The vertical red line indicates γ = 13/3, the naive expectation for a GWB produced by a GW-driven SMBHB population (Phinney 2001).