Data in CMS plots carry error bars that serve as a “best estimate”
of the variance of the Poisson distribution that governs the expected
observations for each observable.
Systematic uncertainties are assumed to follow Gaussian or
log-normal probability distributions.
Collision-based uncertainties come from the luminosity and pileup
calculations.
Detector-based uncertainties come from corrections to the energy
response for different physics objects.
Scale factor-based uncertainties come from the calculation methods
used to measure efficiencies for various reconstruction, identification,
isolation, and trigger algorithms.
Analysis methods also bring in uncertainties that need to be
determined by the analysis team.