I have to admit I'm a little confused- do you want to do the blood spatter project, or a different project?
Assuming that you want to do the spatter project, variables that you could test might include height of the 'origin' of the spatter, or the placement of the sponge. For example, you might investigate the differences in spatter patterns when the sponges are placed against the wall, as opposed to in the middle of the corner. Or you could test for differences with different sizes of the sponge, or even heights of sponges; there are lots of possibilities! What are you interested in investigating?
If you want to do another project, you could look through the Project Ideas, or even look through the forums for things other people are doing. There are tons of awesome ideas and projects out there- pick something that you'll enjoy and that you're interested in.
Hope this helped, and good luck!
Nature uses only the longest threads to weave her patterns, so that each small piece of her fabric reveals the organization of the entire tapestry.