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VALIDATION.md

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+Validation checklist (C# vs Python)
+
+Goal: verify output parity for the same ISIN and inputs.
+
+1) Build inputs
+   - Use the same ISIN and the same prompts in both apps.
+   - Set the same number of simulations.
+   - Use the same dividend flag (s/n).
+   - Optional: set the same RNG seed in Python.
+
+2) Compare core outputs
+   - Correlation matrix and volatility values.
+   - Fair value output line (value and date).
+   - Percent "Sopra/Sotto" and average gain/loss.
+   - Valore atteso.
+
+3) Edge cases
+   - Ask = 0 and Bid > 0 (reference price changes to Bid).
+   - Ex-date warning when DaysToExDate > DaysToObs and last ExDate > last ObsDate.
+
+Notes
+- Monte Carlo output is stochastic; use a seed to reduce variance.
+- If parity differs, compare intermediate inputs printed in the tables.