  Readout errors are a significant source of noise for near term quantum
computers. A variety of methods have been proposed to mitigate these errors
using classical post processing. For a system with $n$ qubits, the entire
readout error profile is specified by a $2^n\times 2^n$ matrix. Recent
proposals to use sub-exponential approximations rely on small and/or
short-ranged error correlations. In this paper, we introduce and demonstrate a
methodology to categorize and quantify multiqubit readout error correlations.
Two distinct types of error correlations are considered: sensitivity of the
measurement of a given qubit to the state of nearby "spectator" qubits, and
measurement operator covariances. We deploy this methodology on IBMQ quantum
computers, finding that error correlations are indeed small compared to the
single-qubit readout errors on IBMQ Melbourne (15 qubits) and IBMQ Manhattan
(65 qubits), but that correlations on IBMQ Melbourne are long-ranged and do not
decay with inter-qubit distance.
