Interaction design: When to check a grouping checkbox

[Tarzan]

Hi!

I have a form where a number of options with checkboxes are grouped (group 1, group 2, group 3 etc.)
You can select items in each group individually. But then I also have a "grouping" checkbox for each group. If you select the checkbox for "Group 1" the whole group will be selected and vice versa.

My question is, what state should the grouping checkbox be in when only a few options are selected - checked or unchecked?
I've seen a few javascripts where the grouping checkbox is checked only when ALL checkboxes within the group are checked, so that the grouping checkbox is an indication on that the whole group is selected.
I'm more inclined to think that the grouping checkbox should tell the user that "some of the options from this group are selected". In this case, the checkbox is checked when one or more options are selected.

What would you prefer?

[misson]

The convention is only checking the group checkbox when all items are checked. Unless you have a very good reason to go against convention ("it makes more sense to me" isn't a good enough reason, because you are the minority), don't do it.

The better solution (which is tricky, but possible) is to implement a tri-state checkbox to indicate that no, some or all items are selected.