We've used an adapted version of Bloom's Taxonomy. The difficulty level links to the grade students are aiming for. It is, of course, very difficult to apply the generic GCSE grading criteria to individual questions, but we think this is a good compromise. We have used a similar system for our Key Stage 3 questions and the IGCSEs.
    

One dot questions (•) are low demand

One dot questions are primarily low level questions requiring simple recall, labelling, naming or describing. There may be some simple calculations or extraction of data involved from bar charts or simple tables.

One dot questions are for all students.
    

Two dot questions (••) are moderate demand

Two dot questions are more conceptually challenging or more complicated questions involving some synthesis or processing of information, or simple calculations. These will involve using a formula and students might need to perform a simple manipulation. Graph work involves some extraction of data and reading values.

In our KS3 content, two dot questions are for students at level 4 or above.
In our GCSE content, two dot questions are for students aiming for grades 3 or above.
    

Three dot questions (•••) are high demand 

Three dot questions are much more conceptually challenging, requiring synthesis and processing of information, evaluation of information, or complex calculations. These questions might contain multiple step calculations, possibly involving a formula. Students might need to manipulate the formula or convert the units. The data for the answer might need to be processed before use. Graph work involves extraction of multiple pieces of data and identifying values from line graphs. Some extrapolation might be required.

In our KS3 content, three dot questions are for students at level 6 or above.
In our GCSE content, three dot questions are for students aiming for grades 5 or above.
  

Four dot questions (••••) are extension questions

Four dot questions are the most difficult in Educake. In addition to the requirements of a three dot question, students might need to link concepts in order to answer a four dot question.

In our KS3 content, four dot questions are for students at level 8 or above.
In our GCSE content, four dot questions are for students aiming for grades 7 or above.

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