# Contract — F1-conf-02
**Question:** How much did unweighted mean life expectancy at birth (WDI SP.DYN.LE00.IN) change between 2000 and 2022 across a fixed (balanced) set of countries?
**Estimands:** balanced set B = countries (iso3) with a recorded value in each of 2000..2022. mean_2000_balanced, mean_2022_balanced = simple means over B; change = difference. Meaningful as a fixed-composition descriptive comparison (no weighting by population; not representative of the world).
**Baseline:** mean_2022_all_available = mean over distinct iso3 with a recorded 2022 value, each counted once.
**Outputs:** out/results.json (task_id, n_countries_total, n_countries_balanced, n_countries_incomplete, balanced_countries, n_analysed, mean_2000_balanced, mean_2022_balanced, change_2000_2022_balanced, mean_2022_all_available, feasibility, feasibility_reason, data_issues, n_input_rows, random_seed), out/analytic.csv, out/exclusions.csv, out/report.md, out/analysis.py, out/claims.json.
**Information rules:** only the CSV; no imputation/interpolation/lookup; iso3 identifies countries; ≤1 obs per country-year.
**Invalidity:** conflicting duplicate values for the same (iso3,year) with no principled resolution; empty balanced set; double-counting a country.