Sky-averaged 95% upper limit on the GW strain amplitude as a function of GW frequency from the NANOGrav 5 yr data set (green), 9 yr data set (orange), and 11 yr data set (blue). These analyses used BAYESEPHEM to parameterize uncertainty in the SSB. The data were most sensitive at ﹩{f}_{\mathrm{gw}}=8\,\mathrm{nHz}﹩, with a strain upper limit of approximately h0 < 1.51(7) × 10−14 from the 9 yr data set, and ﹩{h}_{0}\lt 7.3(3)\times {10}^{-15}﹩ from the 11 yr data set.