  We investigate projection methods, for evaluating a linear approximation of
the value function of a policy in a Markov Decision Process context. We
consider two popular approaches, the one-step Temporal Difference fix-point
computation (TD(0)) and the Bellman Residual (BR) minimization. We describe
examples, where each method outperforms the other. We highlight a simple
relation between the objective function they minimize, and show that while BR
enjoys a performance guarantee, TD(0) does not in general. We then propose a
unified view in terms of oblique projections of the Bellman equation, which
substantially simplifies and extends the characterization of (schoknecht,2002)
and the recent analysis of (Yu & Bertsekas, 2008). Eventually, we describe some
simulations that suggest that if the TD(0) solution is usually slightly better
than the BR solution, its inherent numerical instability makes it very bad in
some cases, and thus worse on average.
