I am always confused when reading about correlation between uncertainties. I have some basic questions: 1. Can statistical and systematic uncertainties be correlated? [Well they can but I don't understand how this can be interpreted] 2. What are correlations telling us ? obviously a correlation is not giving us any relation [or causation]. 3. One example which has confused me: the transverse momentum of charged particles in ATLAS for example can be measured by looking at the charged particle trajectories within the magnetic field. The magnetic field's uncertainty [itex]\sigma_B[/itex] is then common for all particles, and this makes their transverse momenta measurement correlated. How does this affect the analyses?