Abstract: The use of administrative data is increasingly common in transgender research, providing a specific perspective on measuring transgender populations. This study examines how measures of transgender populations using administrative data capture different sociodemographic characteristics over time and around gender transitions. Using population-wide data from Sweden between 2005 and 2019, we compare sociodemographic characteristics with three different measurement strategies to identify the transgender population. The basis of these measurements includes the following markers of trans experiences: gender dysphoria diagnosis, medical treatment, and legal sex change. Our analytical approach involves two regression-based methods: examining sociodemographic differences between the general population and transgender samples over time and performing an event history analysis. Our main findings suggest that when researchers examine transgender demographics or compare results across studies, they should consider three key factors. First, significant sociodemographic changes occur over two historical time points for the transgender population, but these do not depend on the measurement used. Second, in an event history study, the choice of measurement can influence the results. Last, sex assigned at birth influences the differences observed in measurements during an event history analysis.