Use nullable integer

qiskit_experiments/curve_analysis/curve_analysis.py

@@ -364,12 +364,7 @@ def _run_curve_fit(
         # Create convenient function to compute residual of the models.
         partial_residuals = []
         valid_uncertainty = np.all(np.isfinite(curve_data.yerr.to_numpy()))
-        for i, sub_data in list(curve_data.groupby("model_id")):
-            try:
-                m_id = int(i)
-            except ValueError:
-                # Probably invalid data which doesn't belong to any fit model.
-                continue
+        for m_id, sub_data in list(curve_data.groupby("model_id")):
             if valid_uncertainty:
                 nonzero_yerr = np.where(
                     np.isclose(sub_data.yerr, 0.0),
@@ -516,11 +511,7 @@ def _run_analysis(
             fit_curves = []
             formatted = curve_data.filter(like="formatted", axis="index")
             columns = list(curve_data.columns)
-            for i, sub_data in list(formatted.groupby("model_id")):
-                try:
-                    m_id = int(i)
-                except ValueError:
-                    continue
+            for m_id, sub_data in list(formatted.groupby("model_id")):
                 name = self._models[m_id]._name
                 xval = sub_data.xval.to_numpy()
                 if len(xval) == 0:

qiskit_experiments/library/characterization/analysis/ramsey_xy_analysis.py

@@ -15,6 +15,7 @@
 from typing import List, Union
 
 import lmfit
+import pandas as pd
 import numpy as np
 from uncertainties import unumpy as unp
 
@@ -489,7 +490,7 @@ def _format_data(
 
         # Invalidate model_id because these are index of raw Ramsey curves.
         # Fit models are defined based on the phase data.
-        formatted_data.model_id = "n/a"
+        formatted_data.model_id = pd.NA
         out = formatted_data.append_list_values(
             other=phase_data,
             prefix="formatted",