I have finished the user experiment on Tuesday, and started the first phase of data analysis.
First of all, I computed the accuracy of all questions to get an overview of the task performance. Then, I did the Friedman ANOVA on the subjective feedback to validate my experiment design (i.e. difficulty for each level).
In the next step, I'll continue to analyse the features I've tested on the external dataset, and focus on the letter specific features.