Common assumption: attribute values uniformly distributed.
Example: Consider the query
select * from Parts where colour='Red'
If V(colour,Parts)=4, r=1000 ⇒ |σcolour=red(Parts)|=250
In general, | σA=c(R) | ≅ rR / V(A,R)
Heuristic used by PostgreSQL: | σA=c(R) | ≅ r/10