The probability of the nth sample under the kth model Θ k (for k ∈ {1, 2}) is denoted p k (x n k ; Θ k ).
in the parallel data setting, the probability of x n k under its model is: 1 whereas in the non-parallel data setting, the probability is defined as: Θ 2 ) = p(e n ; Θ 2 ).