Today, I attended the regular Monday group meeting in the morning. I think what Dan talked about in his presentation made me rethink about the validity of my experiments, particularly the difficult levels. In my experiments, the primary task remains on the same level, and the reason is that it may be very hard if I shrink the range further down -- but still needs to be proved. Meanwhile, the secondary task has three levels. In the pilot study, I should focus on analyzing the data and to find evidence that participants indeed feel Level 3 is harder than Level 2.
I also organized the steps of each tasks and wrote a detailed description of my experiments containing screenshots of UI and timeline of the experiment. It is in CVS.