Independant variable for Asprin pill titration with NaOH solution
Independent variable means the variable which you can change easily. Direct titration aspirin is not a good idea as aspirin hydrolyzes into salicylic acid. There are a lot of non-sense high school/college level experiments doing direction titration and ignoring the side reaction of hydrolysis.

The proper procedure of aspirin analysis is by back-titration with NaOH and taking hydrolysis into account. Ask your teacher about it by showing him/her this procedure Back titration of aspirin

Coming back to independent variables...Just now you have another independent variable...comparison of two different methods of titration (direct vs. back). Think of other independent variables and share in your comments.