Common assumption: attribute values uniformly distributed.

Example: Consider the query

select * from Parts where colour='Red'

If   V(colour,Parts)=4,   r=1000  ⇒  |σ_colour=red(Parts)|=250

In general, | σ_A=c(R) |  ≅  r_R / V(A,R)

Heuristic used by PostgreSQL: | σ_A=c(R) |  ≅  r/10