Experimental Design: The design of any task that aims to describe or explain the variation of information under conditions that are hypothesized to reflect the variation. This is like planning how you'll investigate your crime scene before you get there.
Data Collection: The process of gathering and measuring information on variables of interest. This is like picking up clues at your crime scene.
Qualitative Data: Information describing qualities or characteristics that can't be measured numerically. Like noting if your suspect seems nervous - it's important info but not something you can put a number on!