An arbitrary qubit is represented as $\alpha|0\rangle+\beta|1\rangle$ with $|\alpha|^2+|\beta|^2=1$. If we know either $\alpha$ or $\beta$, the state can be completely identified. The 'arbitrariness' of an arbitrary qubit thus can be said to be '1'. For a known state this is '0'. Now if we consider an arbitrary two qubit state, the total number of variables are 3 (actually 4 but 1 less due to normalization). If any two of these variables are equal or one of them is known, the state is still arbitrary but from a given restricted class. We can say that the 'arbitrariness' of this quantum state is something between 0 and 1. Can anyone suggest a measure for the arbitrariness of this quantum state? I want to extend this measure to higher order states. Can anyone suggest any reference for this?