  State preparation and measurement (SPAM) errors limit the performance of
near-term quantum computers and their potential for practical application. SPAM
errors are partly correctable after a calibration step that requires, for a
complete implementation on a register of $n$ qubits, $2^n$ additional
measurements. Here we introduce an approximate but efficient method for
multiqubit SPAM error characterization and mitigation requiring the classical
processing of $2^n \! \times 2^n$ matrices, but only $O(4^k n^2)$ measurements,
where $k=O(1)$ is the number of qubits in a correlation volume. We demonstrate
and validate the technique using an IBM Q processor on registers of 4 and 8
superconducting qubits.
