Personly I would say having an overview is the best for the exam so read the text book from start to finish then after knowing what happens and why.. then try and learn and remember factual contex (% and numbers ect but mainly just round them up so instead of trying to remember 4,946,643 votes just say 5million) our teacher said the marker gets around10p per paper they mark and they have to mark hundreds so just keep that in mind they don't remember everything and arn't going to not give you a mark for not getting the exact number