Before answering your first question, I thought it might be helpful to present the following: http://www.empirisoft.com/support/sh...52&postcount=2
Given your equation it doesn't quite matter too much to switch out the randbetween function with RAND(); however it may help to know that the prior doesn't quite work as of now.

In regards to your question, MediaLab can reference any variables on the responses.xls grid by using angle brackets within other locations of the experiment. More information can be found here:
http://www.empirisoft.com/medialab/h...ted_scores.htm

To answer your second question - currently DirectRT does not support excel functionality in the manner presented; however we hope to eventually have our 2010 version working with this feature.
The first strategy should work. If you need help programming this experiment, feel free to let us know.

By using both custom items and MediaLab's advanced features, just about anything can be created.