From the paper: &quot;To account for the search background noise varying across the target signal space, candidate and background events are divided into three search classes based on template length. The right panel of Fig. 4 shows the background for the search class of GW150914. The GW150914 detection- statistic value of ρˆ_c = 23.6 is larger than any background event, so only an upper bound can be placed on its false alarm rate. Across the three search classes this bound is 1 in 203 000 years. This translates to a false alarm probability &lt; 2 × 10^−7, corresponding to 5.1σ. A second, independent matched-filter analysis that uses a different method for estimating the significance of its events [85,86], also detected GW150914 with identical signal parameters and consistent significance&quot; (<a href="https:&#x2F;&#x2F;dcc.ligo.org&#x2F;LIGO-P150914&#x2F;public" rel="nofollow">https:&#x2F;&#x2F;dcc.ligo.org&#x2F;LIGO-P150914&#x2F;public</a>). Take a look at Figure 4 as well.<p>In case you&#x27;d like to dig deeper, the 85 and 86 mentioned are:<p>[85] K. Cannon et al., Astrophys. J. 748, 136 (2012).<p>[86] S. Privitera, S. R. P. Mohapatra, P. Ajith, K. Cannon, N. Fotopoulos, M. A. Frei, C. Hanna, A. J. Weinstein, and J. T. Whelan, Phys. Rev. D 89, 024003 (2014),