An earlier posting identified four basic types of case study design. Two of these have embedded units of analysis: what does this mean? Yin (2009, p.50) explains with the example of a public programme (the single case) which funds a large number of projects (the embedded units). See also his more comprehensive guidance (pp.29-33) about defining what the case is and choosing the unit of analysis.
Yin (2010, pp.82-87) offers another viewpoint in a discussion about ‘data collection units’ and their relationship to the main topic of a study. He notes that the data collection unit is usually the unit of analysis, though there can be complicated situations when it is not. He suggests that “most qualitative studies have more than one level of data collection unit” and that these “are likely to fall within a nested arrangement: a broader level (eg a field setting) that contains or embeds a narrower level (eg a participant in the setting)”.His Exhibit 4.1 lists examples of a topic and two levels of data collection unit.
YIN, R. K. 2009. Case study research: Design and methods, Sage publications, INC.
YIN, R. K. 2010. Qualitative research from start to finish, The Guilford Press.