Researchers and evaluators interested in positive youth development (PYD) programs seek to understand what works for what youth in what ways. Typically, measurement and analysis are framed by ergodic theorems, which assume homogeneity of individuals and stationarity in individuals’ developmental pathways. However, such commonality (homogeneity and stationarity) does not characterize all developmental data sets. Here, we interrogate ergodicity assumptions using data from the Compassion International (CI) Study of PYD in El Salvador. We assessed 1205 youth (M_age = 13.03; 49.8% female; 67.1% urban), 51% of whom were enrolled in CI-supported programs, regarding links among youth strengths, contextual resources, and program outcomes. We replicated analyses across specific subgroups (urban-rural, gender, and CI-enrollment status) to determine which findings were robust (demonstrating commonality) and which findings were unique (demonstrating specificity). Instances of commonality and specificity characterized many relations. Implications for designing and enhancing developmental programs are discussed.