Stylometric Comparison of Professionally Ghost-Written and Student-Written Assignments

We report a stylometric investigation of a portfolio of 20 assignments submitted by an individual student over two consecutive academic years. This investigation followed a formal disciplinary investigation which had identified that eight of the assignments had been ghostwritten, with seven of those showing explicit ghost-writer ID information and three of those showing ID information from the same commercial provider. The stylometric investigation involved a conventional word and bigram frequency analysis and a prototype word complexity analysis. The word and bigram analysis identified four consistent groups of assignments, which associate other assignments with the eight known to have been ghost-written, indicating that those were probably also ghost-written. One of those groups comprises the three assignments from the same provider, plus another assignment, implying that the provider has a ‘house style’ and that the other assignment also came from that provider. The prototype analysis clearly categorised the core members of two of those same groups, including the group from the identified provider, adding further weight those associations. More generally, this investigation shows that it is possible to categorise assignments according to aspects of writing style: we would have obtained the same groups even if we had not possessed the ghost-writer ID information. Where such consistent groups are identified it implies, on balance of probabilities, multiple authorship of assignments and that the student concerned cannot have written all the
submitted assignments and that some were ghost-written.