  Implicit-Explicit (IMEX) schemes are widely used for time integration methods
for approximating solutions to a large class of problems. In this work, we
develop accurate a posteriori error estimates of a quantity of interest for
approximations obtained from multi-stage IMEX schemes. This is done by first
defining a finite element method that is nodally equivalent to an IMEX scheme,
then using typical methods for adjoint-based error estimation. The use of a
nodally equivalent finite element method allows a decomposition of the error
into multiple components, each describing the effect of a different portion of
the method on the total error in a quantity of interest.
