Extended essays are marked by external assessors (examiners appointed by the IB) on a scale of 0 to 36. There are "general" and "subject-specific" criteria, at a ratio of 2:1 (24 possible marks for the general criteria and 12 marks for the subject-specific one). The total mark is converted into a grade from A to E. A similar system is used for theory of knowledge and students can gain up to 3 points for the diploma based on the grades achieved for EE and TOK. A student who scores an E on either the extended essay or TOK essay will be automatically failed and no longer eligible to receive the IB Diploma (EE Subject Guide p15). 
Please read the reports before reading the essays. The reports contain valuable information directly from IB examiners on what was successful and what was not. Also remember that just because something was successful in an exemplar, it does not mean that this is how the essay must be written. There are many different ways to get a good mark on your extended essay and it is important that you find the approach that best suits the paper you are writing. The exemplars below are only meant to clue you in on the type of analysis examiners like rather than providing a stylistic from that you must follow.