Correlation between E_p and the 40–700 keV fluence for a sample of 108 bright GRBs with well determined values. The dashed line shows the best-fitting power law, with index 0.21 ± 0.06, while the dotted lines include the 1-σ region, where σ = 0.13 is the extrinsic scatter. Inset: distribution of the normalised scatter. The dashed line shows the standardised normal distribution.