Aside from the validity and reliability of marking, there are other biases. These include those associated with the student (sex, race, physical attractiveness), with the marker (personal prejudice, teacher expectation), the context in which the work is marked (carryover effects, mood) and the essay itself (handwriting). Studies also revealed a "slide effect" by which essays marked earlier in the sequence obtained higher marks than those marked nearer the end. This has profound effects if the essays are marked in alphabetical order.
Mark Griffiths
Psychology division
Nottingham Trent University