Refs #21401. Propagate y value errors for _elt workspace[C

Framework/PythonInterface/plugins/algorithms/WorkflowAlgorithms/ElasticWindowMultiple.py

@@ -11,6 +11,9 @@ from mantid.kernel import *
 from mantid.api import *
 
 
+import numpy as np
+
+
 def _normalize_by_index(workspace, index):
     """
     Normalize each spectra of the specified workspace by the
@@ -19,15 +22,23 @@ def _normalize_by_index(workspace, index):
     @param workspace    The workspace to normalize.
     @param index        The index of the y-value to normalize by.
     """
+    number_of_histograms = workspace.getNumberHistograms()
+
+    for idx in range(0, number_of_histograms):
+        y_values = workspace.readY(idx)
+        y_errors = workspace.readE(idx)
+
+        # Normalise y values
+        scale = np.reciprocal(y_values[index])
+        y_values_normalised = scale * y_values
 
-    num_hist = workspace.getNumberHistograms()
+        # Propagate y errors: C = A / B ; dC = sqrt( (dA/B)^2 + (A*dB/B^2)^2 )
+        a = (y_errors*scale)
+        b = (y_values*y_errors[index]*(scale ** 2))
+        y_errors_propagated = np.sqrt(a ** 2 + b ** 2)
 
-    # Normalize each spectrum in the workspace
-    for idx in range(0, num_hist):
-        y_vals = workspace.readY(idx)
-        scale = 1.0 / y_vals[index]
-        y_vals_scaled = scale * y_vals
-        workspace.setY(idx, y_vals_scaled)
+        workspace.setY(idx, y_values_normalised)
+        workspace.setE(idx, y_errors_propagated)
 
 
 class ElasticWindowMultiple(DataProcessorAlgorithm):