  Journal ranking is becoming more important in assessing the quality of
academic research. Several indices have been suggested for this purpose,
typically on the basis of a citation graph between the journals. We follow an
axiomatic approach and find an impossibility theorem: any self-consistent
ranking method, which satisfies a natural monotonicity property, should depend
on the level of aggregation. Our result presents a trade-off between two
axiomatic properties and reveals a dilemma of aggregation.
