term frequencies in sharded environment

[Massimo Schiavon]

Seems that when I submit a query in a sharded environment the idf 
component of the scoring formula takes into consideration the local 
terms frequencies (local to the single shard index). The effect of that 
is that the calculation is correct only if the distribution terms in the 
shards is balanced.

Are there any way to avoid that? Perhaps by using cumulative frequencies 
in the calculation? Anything else?

Regards

Massimo