They either have focused on scenarios where the policy is able to explore and collect more data (Degris et al., 2012; Riedmiller, 2005) such as learning online from an outdated replay buffer (e.g. (Munos et al., 2016)), or have performed off-policy policy evaluation (Farajtabar et al., 2018; Jiang and Li, 2016;Precup, 2000; Thomas and Brunskill, 2016).
we learn a policy entirely offline, from a fixed batch of data, with no ability to explore.