  A good translation should be faithful to the source and should respect the
norms of the target language. We address a theoretical puzzle about the
relationship between these objectives. On one hand, intuition and some prior
work suggest that accuracy and fluency should trade off against each other, and
that capturing every detail of the source can only be achieved at the cost of
fluency. On the other hand, quality assessment researchers often suggest that
accuracy and fluency are highly correlated and difficult for human raters to
distinguish (Callison-Burch et al., 2007). We show that the tension between
these views is an instance of Simpson's paradox, and that accuracy and fluency
are positively correlated at the level of the corpus but trade off at the level
of individual source segments. We further suggest that the relationship between
accuracy and fluency is best evaluated at the segment (or sentence) level, and
that the trade off between these dimensions has implications both for assessing
translation quality and developing improved MT systems.
