Abstract: Privacy is a necessary requirement for voting. Without privacy, voters can be forced to vote in specific ways, and the forcing party can check their compliance. But offering privacy does not suffice: if a voter can reduce her privacy, an attacker can force her to do so. We introduce the notion of choice groups as a measure of privacy, and provide formal definitions. We illustrate how this notion can be used to better understand privacy concerns in voting systems, and how the notion of choice groups is formalised to quantify privacy. By combining this with models of various degrees of voter collaboration, we can quantify the privacy under a voter\'s control.