  We say that two (or more) state assignments for one and the same quantum
system are compatible if they could represent the assignments of observers with
differing information about the system. A criterion for compatibility was
proposed in [Phys. Rev. A 65, 032315 (2002)]; however, this leaves unanswered
the question of whether there are degrees of compatibility which could be
represented by some quantitative measure, and whether there is a
straightforward procedure whereby the observers can pool their information to
arrive at a unique joint state assignment. We argue that such measures are only
sensible given some assumption about what kind of information was used in
making the state assignments in the first place, and that in general state
assignments do not represent all of the information possessed by the observers.
However, we examine one particular measure, and show that it has a
straightforward interpretation, assuming that the information was acquired from
a particular type of measurement, and that in this case there is a natural rule
for pooling information. We extend this measure to compatibility of states for
k observers, and show that the value is the solution to a semidefinite program.
Similar compatibility measures can be defined for alternative notions of state
compatibility, including Post-Peierls (PP) and Equal Support (ES)
compatibility.
