  Recently, \citet{SuttonMW15} introduced the emphatic temporal differences
(ETD) algorithm for off-policy evaluation in Markov decision processes. In this
short note, we show that the projected fixed-point equation that underlies ETD
involves a contraction operator, with a $\sqrt{\gamma}$-contraction modulus
(where $\gamma$ is the discount factor). This allows us to provide error bounds
on the approximation error of ETD. To our knowledge, these are the first error
bounds for an off-policy evaluation algorithm under general target and behavior
policies.
