So are you assuming that the dirt in the glassware is staying the same (or haiving the same affect) throughout the experiment? What then happens if the glassware becomes "cleaner" throughout the experiment?
Also, why do you not include having dirty glassware as poor technique, surely part of the technique is to have clean glassware!
I think I just see too many students who blame everything on "dirty glassware" before they will even think about the accuracy of a pipette or burette, parallax error reading a pipette or burette, accuracy of their balance, accuracy of their density graph
I think that having air lock in your burette is not as valid a source of error as these, what if the airlock disappears during the experiment, how do you factor this in?
Errors is always an intersting topic