# Contract — F1-conf-02
**Question:** How much did life expectancy at birth change 2000→2022 across a fixed (balanced) set of countries?
**Estimands:** for each year, the unweighted mean of life expectancy over countries (iso3) with a recorded value in every year 2000–2022, each counted once; key quantities are the 2000 and 2022 balanced means and their difference, plus the balanced-set composition. Meaningful if each country contributes one value per year and missing values are not imputed. Baseline: naive mean over all distinct countries with a 2022 value.
**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 (6 sections); out/analysis.py; out/claims.json.
**Information rules:** use only data/life_expectancy_panel.csv; no imputation/interpolation/lookup; country = iso3; ≤1 observation per country-year.
**Invalidity:** conflicting duplicate values for the same iso3-year (no rule to choose); empty balanced set; double-counting a country.