populating a field in a parent form based on a field in a subform

populating a field in a parent form based on a field in a subform

I have a parent form with a subform, and I know how to have fields on the subform populate based on the parent form. But is the reverse possible? That is, can a field on the subform be used to update a field on the parent form, upon either user entry or submission of the subform? If so, how would I script this?