Search result: Costa Rica middle class
Use Banerjee and Duflo (2008) for the 3 arguments of the importance of middle class.
Use Lora and Fajardo (2011) for the categories of objective criteria of middle class.
Adopt the absolute measure as the first approach (overlapping with the subjective measure).
Apply the absolute method to Costarican data (Latinobarometro).
Then, check the descriptive data, contrasting against the poor and the rich.