How different or distinct should the disciplines be in order for the proposal to be considered eligible?
It depends entirely on the proposal. Many disciplines, such as those which are interdisciplinary and applied, include elements from other scientific fields. However, they also have their own unique approaches and expertise. In order to be competitive the applicant must demonstrate how the collaboration will rely on these differences to allow something to happen that otherwise could not have occurred within the confines of the PI’s discipline alone.
