The lack of valid and reliable measures of firm-level vertical and horizontal internationalization is impeding the development and testing of hypothesized relationships between these respective dimensions of internationalization and a range of important MNE characteristics, actions, and effects. Through a series of qualitative and quantitative studies using data collected from senior MNE executives (total N=3,146), we develop and validate a scale to measure both vertical and horizontal firm-level internationalization. Subscales for each type of internationalization prove to be unidimensional, reliable, temporally stable, and to have predictive, cross-cultural, cross-sectoral, and discriminant validity
