"When data are collected from multiple individuals in a group, the individual data are considered nested within that group."
O'Dwyer, L. M., & Parker, C. E. (2014). A primer for analyzing nested data: Multilevel modeling in SPSS using an example from a REL study. Institute of Education Sciences U.S. Department of Education. https://ies.ed.gov/ncee/edlabs/regions/northeast/pdf/REL_2015046.pdf