In statistics, specifically survival analysis, one often talks about the number of survivors and decedents in a cohort. What would be an appropriate word to describe the two groups?
The APA Style site recommends the terms participants or subjects, noting:
...for more than 100 years the term subjects has been used within experimental psychology as a general starting point for describing a sample, and its use is appropriate.
So survivors, decedents, and dropouts as a whole constitute the initial subjects of the study; the survivors and decedents who remained are the subjects who are entered into the analysis.