In both forms of Directed Independent Study, both Dissertations and Electives, you work with a supervisor whom you meet regularly throughout the semester. There is less structure than in standard modules. You are expected to be self-motivated and to initiate lines of enquiry yourself.
Dissertation projects require you to work out in advance the course of study you wish to pursue. There is no corresponding formal requirement for taking Electives, although in preparation for Electives, as much as for dissertation projects, it is advisable to do substantial reading in advance of the formal commencement of study.
Working on a dissertation topic of your own devising, following a self-directed course of study, offers a chance to develop some of the skills needed for independent research, and can be specially rewarding. At the same time, there are dangers attached to dissertation projects and especially to PHI9YY projects. One common pitfall is allowing the more immediate pressures from other modules (reading for seminars, essay writing, exam preparation) to reduce the time you spend on your dissertation. This can be especially acute on PHI9YY (2 semester) projects. It is not unusual for students to make insufficient progress in the Autumn semester, rush to complete in time for the submission deadline, and as a result submit a poor dissertation. In the absence of formal deadlines other than the date for submitting the dissertation you need to manage your time well.
Another danger attached to dissertation projects is aiming to be far too ambitious. Projects have to be feasible in the time available. So they have to be fairly sharply focused. If you think of writing a dissertation as throwing off the disciplines and constraints of a standard taught module and allowing your spirit to fly then it is probably not a good idea to undertake a dissertation project. Part of the point of doing a dissertation is to gain experience in thinking about a topic independently, but you still need to think effectively and that will require you to engage with a body of literature that you understand, be responsive to guidance from your supervisor, and take the time to write in a manner that displays a good grasp of your topic.
Especially careful thought should be given to whether you wish to undertake a PHI9YY (44-credit) Dissertation. These projects attract 44 credits. The grade you receive counts towards degree classification as much as two standard taught modules and is based on a single item of work—the dissertation. To prepare for such projects you need to think about them well before the April deadline for submission of proposals. Ideally you would be giving thought to possible projects in the winter vacation between your fifth and sixth semesters (or the equivalent if you are a February or later entrant.)
The best indication of whether you should undertake a dissertation project is whether you have a good well worked out proposal. The more you have to depend on members of staff to tell you what you could do and how you could do it, the less likely it is that a dissertation is right for you.