Uncertainties in CMS analyses: Key Points

Introduction

Uncertainties are always divided into statistical and systematic components
Many precision measurements also divide out luminosity and/or theory uncertainties

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.

The available 2016 corrections and a summary website can be accessed from the Open Data Portal (a “record” page for the corrections is forthcoming).
Mandatory corrections such as pileup and jet energy corrections are provided.
Scale factors for many reconstruction and identification algorithms are also provided.
The summary webpage is the best reference to understand the names, inputs, and outputs of the corrections.

The correctionlib software provides a method to open and read corrections in the common JSON format.
The evaluate method takes in a correction’s required inputs and returns a correction value.
For event-weight corrections, save shifted event weight arrays to create shifted histograms.
ROOT histograms can be created simply from arrays of data and weights for statistical analysis.