In the case of summative assessments, I always use rubrics that have been given to students in advance so that they know what content or performance is expected from them by the teacher. This of course makes the assessment and evaluation that they receive fair since they know what is expected. I believe that a great means of assessing students should be based on four qualities: conclusions/logical flow of argument, organization, research, and format. Format would encompass the creative nature of the product as well as the language, grammar and spelling (unless there’s a good reason grammar and spelling are unconventional in the case of rap songs, for example). These are three of the main skills that students should be learning to develop in the course of a social studies class.
Here is an example of a rubric that I used to assess students on the creation of medieval weapons that would have been used in the late Middle Ages: