  In each of the last five years, a few dozen empirical studies appeared in
visualization journals and conferences. The existing empirical studies have
already featured a large number of variables. There are many more variables yet
to be studied. While empirical studies enable us to obtain knowledge and
insight about visualization processes through observation and analysis of user
experience, it seems to be a stupendous challenge for exploring such a huge
variable space at the current pace. In this position paper, we discuss the
implication of not being able to explore this space effectively and
efficiently, and propose means for addressing this challenge.
