Often these are worth two module credits and can disproportionately weigh in the balance when it comes to determining a final degree award. Typically dissertations are blind double marked. So that each marker judges the work independently and only then discusses with a colleague, the definitive mark to be awarded.

Do dissertations get graded?

Dissertations are not graded, and the GPA includes only course grades. For the most part, people do not care about course grades in Ph. D. programs once you've completed your dissertation.

Does your supervisor Mark your dissertation?

The assessment is double-marked, once by the supervisor and once by a second marker. Any difference in marks are then discussed and agreed. The assessment would be marked by someone not related to your project at all, and it may also be double-marked by someone else.