Data missing by design included participants who did not join the study until they enrolled at a participating school, and those who left after being promoted out of the school. RSiena handles this type of missingness by composition change (Huisman & Snijders, 2003). Other types of missingness are handled in RSiena with the objective of minimizing their effects on the analysis (Huisman & Steglich, 2008; Snijders et al., 2010).
