For example take the hypothesis "people who listen to Mozart tend to be intelligent". Is the dependent variable intelligence or if a person listens to Mozart? For the sake of the study is it arbitrary? I say this because I've come across 2 definitions of independent variables:
1)The variable that influences the dependent variable
2)The variable that testers manipulate
So in a situation like this it's arbitrary if a tester looks for people who do or do not listen to Mozart, or alternatively, look for people with a particular level of intelligence.