How do I go about deleting/dissolving groups randomly?

For example, after shattering I get piece0, piece1…. and I want to delete/dissolve one or several of them.

Edit: I tried to enter the following into the group:

piece`int(fit(rand(3),0,1,0,3))

but it only allows me to delete one group and I don't know the expression to select from all groups automatically, rather than manually entering the group number in the last place of the fit expression.

Check this topic:
http://forums.odforce.net/index.php?/topic/9161-primitive-groups-to-pointgroups/ [forums.odforce.net]

There is a OTL that uses expressions you are lookig for.