  We consider two approaches to estimate and characterise the theoretical
uncertainties stemming from the missing higher orders in perturbative
calculations in Quantum Chromodynamics: the traditional one based on
renormalisation and factorisation scale variation, and the Bayesian framework
proposed by Cacciari and Houdeau. We estimate uncertainties with these two
methods for a comprehensive set of more than thirty different observables
computed in perturbative Quantum Chromodynamics, and we discuss their
performance in properly estimating the size of the higher order terms that are
known. We find that scale variation with the conventional choice of varying
scales within a factor of two of a central scale gives uncertainty intervals
that tend to be somewhat too small to be interpretable as 68%
confidence-level-heuristic ones. We propose a modified version of the Bayesian
approach of Cacciari and Houdeau which performs well for non-hadronic
observables and, after an appropriate choice of the relevant expansion
parameter for the perturbative series, for hadronic ones too.
