step,rule,n_before,n_after,n_removed,rationale
1_year_window,keep 2000 <= year <= 2022,439,439,0,study window defined by the task
2_dedup_iso3_year,"keep one row per (iso3, year); duplicates have identical values",439,416,23,each country contributes at most one observation per year (assembly duplicates)
3_drop_missing_value,drop rows with empty life_expectancy,416,415,1,a country-year without a recorded value is missing; no imputation
4_balanced_panel,keep only iso3 with a recorded value in every year 2000-2022,415,345,70,"balanced panel; excluded countries: BGD, CHE, ETH, IND, NGA"