Update README.md

README.md

@@ -213,6 +213,38 @@ For each date:
 `emergence(t) = (sum of average_values up to date t / sum of all average_values) × 100`  
 → Shows cumulative emergence progress as a percentage.
 
+<details>
+<summary><strong>Calculation logic</strong></summary>
+
+## 2. Calculation logic — unified view
+
+### Time-series definition
+
+- Each calculation is performed **per time series**  
+  (currently: *Trial ID*; in the future possibly *Trial ID + Repetition*).
+- All dates belonging to the same time series are processed together.
+
+---
+
+### Data validation
+
+Before any calculation:
+
+- All observations of a given `assessment_type` **must use the same unit**.
+- If mixed units are detected, an **exception is raised**.
+
+---
+
+### Date aggregation
+
+- If multiple observations exist for the same date (e.g. replicates or subsamples):
+  - Values are **averaged first**.
+
+After averaging:
+
+- Only **one value per date** is retained.
+
+</details>
 
 ## Citation Information
 @misc{opheno_dataset_2026