Dive into the data - Analyzing Assessment Data

Every unit I give formative assessments that drive my instruction.  I alter lessons plans, reteach and reassess as needed and seek to, through effective feedback and standards-based grading, help students succeed.

This got me to thinking.....

How do you analyze test data?  

Whether it is a quiz, a summative exam, a project?  What do you do at the end of the quarter, unit, project, etc. to wrap up and dive into the data?

Here's what I do at the end of every unit: https://docs.google.com/viewer?a=v&pid=explorer&chrome=true&srcid=0Bwz1J6fj2c9rY2E1ZThlNGQtODZiMy00MDk2LWE3ZTYtZGU2ZTg0M2MyMzIy&hl=en_US&authkey=CL2rnegD

No one asks me to do it but I am able to gain valuable information about what worked and what didn't.  I know this is like performing a rather detailed autopsy of something that already happened and something I can do nothing about, but it changes my behavior for the next time I complete this unit.

What are your thoughts?  What do you do?  What data do you dive into?