Are PhD students responsible for a lot of general lab work (4 out of 5 days of work)?
It’s unclear what you mean by “general lab work”. What is and is not reasonable work for graduate students can vary significantly between different fields. In general it’s pretty reasonable to expect graduate students to contribute to the general upkeep of a lab or facility, even though it seems “unrelated” and can cut into the time a student would rather spend on other things. I think bringing the money into this is a mistake. She should not be paid extra, since that would essentially be double-dipping with the stipend. Her stipend is in all likeliness not set by the PI, and so I don’t know why that would factor into to the grievance.
