In multilevel models for longitudinal data, the lowest level of data is the specific measurement at a particular time. This lowest level is referred to as "Level-1" data. Each Level-1 measurement is nested within a particular research participant. Davis, M. L., Neelon, B., Nietert, P. J., Burgette, L. F., Hunt, K. J., Lawson, A. B., & Egede, L. E. (2021). Propensity score matching for multilevel spatial data.