Free Tuition, Stratified Pipelines: Four Decades of Administrative Cohorts and Equity in Access to Engineering and Science in an Argentine Public University

Latin American higher education is often portrayed as equitable when tuition is free and access to public universities is formally unrestricted. Yet, growing research shows that massification under tuition-free policies often coexists with strong social and territorial stratification. This article uses four decades of administrative records from a faculty of engineering in north-western Argentina to examine how cohort composition has changed over time. Drawing on 24,133 first-time entrants (1980-2019), we construct a leakage-aware "background census" layer (N1c) harmonising school type, province, and age across legacy systems. We combine descriptive analyses, UMAP+DBSCAN clustering, and a reconstructed macroeconomic panel (inflation, unemployment, poverty, GDP) anchored at entry. All analyses explicitly report structural missingness patterns. Results show that missingness in background variables is historically patterned, declining sharply after the 1990s. Among students with observed data, the share coming from private-especially religious-secondary schools in high-income areas increased from less than half in the 1980s to roughly two-thirds in the 2010s. The catchment area became more local, with the home province gaining weight while distant origins lost ground. Median age at entry remained stable at 19, with persistent right tails of older entrants. Macro-linkage analyses reveal moderate associations between unemployment and older entry age, and between inflation and higher shares of students from interior provinces. We argue that free tuition and open entry have operated within, rather than against, stratified school and residential pipelines. The article illustrates how administrative data can support equity monitoring and discusses implications for upstream school policies and institutional accountability in tuition-free systems.