After a long period of case study research and small-N comparisons, various scholars and projects have started to compare a relatively large range of cases in the citizenship literature. This new research approach has inevitably led to a quantification of the data under study, however, not yet to the creation of a widely accepted citizenship indicator. On the contrary, there are almost as many indices as there are large-N studies (Koopmans, Howard, MIPEX, Janoski, Waldrauch and Hofinger etc.). The emerging pioneer spirit has left surprisingly little space to the question of how useful these indicators are to compare citizenship policies. Do we need so many indicators? Do they all measure the same or different concepts? Do they really measure what they are supposed to measure? While hardly anybody will disagree that we need a more thorough discussion on these questions, it is all the more astonishing that such a debate has been absent so far. For this reason, this paper proposes a discussion of the criterion, content, convergent and construct validity of the existing indicators. Besides the more general claim that we urgently need such a debate, I like to make three further points in this paper: (1) There is evidence that most of the existing indicators measure the same concepts, irrespective of how sophisticated the indicators are. It would therefore be more efficient to stick to simple indicators and avoid projects that require a huge amount of resources for the data collection. (2) Moreover, while some indicators combine information on policy aspects with naturalization rates, keeping apart these two aspects would allow us to make a difference between the output and the outcome of citizenship politics. Isolating policy outcomes is particularly important to study policy effects. (3) While most often naturalization rates are used to study policy outcomes, which is the ratio between the yearly number of naturalisations and the number of foreign residents in a country, I claim that we should focus on rejection rates, i.e. the ratio between the rejected and the submitted applications, that is a more valid way to study policy effects. This paper builds on a EUDO citizenship forum debate: http://eudo-citizenship.eu/citizenship-forum/380-which-indicators-are-most-useful-for-comparing-citizenship-policies
