3. The method according to claim 2 , wherein determining the frequency of use of the expression comprises: calculating a percentage of the occurrences of the expression in a set of predefined sample texts.
3. The method according to claim 2 , wherein determining the frequency of use of the expression comprises: calculating a percentage of the occurrences of the expression in a set of predefined sample texts. 4. The method according to claim 3 , wherein associating each expression with the difficulty index further comprises: calculating a further percentage of the occurrences of the expression in the text, and updating the frequency of use of the expression according to the further percentage.