diff --git a/cardinal/metrics.py b/cardinal/metrics.py
new file mode 100644
index 0000000..542e1c7
--- /dev/null
+++ b/cardinal/metrics.py
@@ -0,0 +1,87 @@
+import logging
+from abc import ABC, abstractmethod
+
+import numpy as np
+
+
+class BaseMonitor(ABC):
+    """A monitor is a metric and a set of utils to record it and monitor it.
+
+    Args:
+        batch_size: If specified, a warning will be issued if batch_size is not correct
+        tolerance: 
+
+    """
+
+    def __init__(self, batch_size=None, tolerance=None):
+        self.batch_size = batch_size
+        self.tolerance = tolerance
+        self.reset()
+
+    def reset(self):
+        self.n_samples = []
+        self.values = []
+
+    def _append_n_samples(self, n_samples):
+        self.n_samples.append(n_samples)
+        if not self.batch_size or len(self.n_samples) <= 1:
+            return
+        this_batch_size = self.n_samples[-1] - self.n_samples[-2]
+        if this_batch_size != self.batch_size:
+            logging.warn(
+                'Batch size of iteration {} is {} which is different'
+                'from the reference batch size {}'.format(
+                    len(self.n_samples), this_batch_size, self.batch_size
+                )
+            )
+
+    @abstractmethod
+    def accumulate(self, n_samples, value):
+        pass
+
+    @abstractmethod
+    def get(self):
+        pass
+
+    def is_stalled(self, n_iter=1):
+        if len(self.values) < n_iter + 1:
+            return False
+        for prev_v, curr_v in zip(self.values[-n_iter - 1:-1], self.values[-n_iter]):
+            if np.abs(curr_v - prev_v) > self.tolerance:
+                return False
+        return True
+        
+
+class ContradictionMonitor(BaseMonitor):
+    """Stores the amount of contradictions along an experiment
+
+    We call contradiction the difference between predictions of two successive
+    models on an isolated test set.
+    """
+
+    """Stores contradiction for a new iteration.
+
+    Args:
+        n_samples : Number of training samples
+        probas_test : Predictions of shape (n_samples, n_classes)
+    """
+    def accumulate(self, n_samples: int, probas_test: np.array):
+        if self.last_probas_test is not None:
+            self.values.append(
+                np.abs(probas_test - self.last_probas_test).sum())
+            self._append_n_samples(n_samples)
+        self.last_probas_test = probas_test
+
+    """Returns the recorded metrics
+    """
+    def get(self):
+        return {
+            "n_samples": self.n_samples,
+            "contradictions": self.values
+        }
+
+    """Reset the metrics for a new experiment
+    """
+    def reset(self):
+        super().reset()
+        self.last_probas_test = None
diff --git a/examples/plot_digits_metrics.py b/examples/plot_digits_metrics.py
index 4d54a0e..01780b7 100644
--- a/examples/plot_digits_metrics.py
+++ b/examples/plot_digits_metrics.py
@@ -26,6 +26,7 @@
 from cardinal.random import RandomSampler
 from cardinal.plotting import plot_confidence_interval
 from cardinal.base import BaseQuerySampler
+from cardinal.metrics import ContradictionMonitor
 
 np.random.seed(7)
 
@@ -147,7 +148,7 @@ def select_samples(self, X):
             train_test_split(X, y, test_size=500, random_state=k)
 
         accuracies = []
-        contradictions = []
+        contradictions = ContradictionMonitor()
         explorations = []
 
         previous_proba = None
@@ -167,13 +168,8 @@ def select_samples(self, X):
             # Record metrics
             accuracies.append(model.score(X_test, y_test))
             explorations.append(compute_exploration(X_train[mask], X_test))
-
-            # Contradictions depend on the previous iteration
-            current_proba = model.predict_proba(X_test)
-            if previous_proba is not None:
-                contradictions.append(compute_contradiction(
-                    previous_proba, current_proba))
-            previous_proba = current_proba
+            contradictions.accumulate(len(selected),
+                                      model.predict_proba(X_test))
 
             sampler.fit(X_train[mask], y_train[mask])
             selected = sampler.select_samples(X_train[~mask])
@@ -181,7 +177,7 @@ def select_samples(self, X):
 
         all_accuracies.append(accuracies)
         all_explorations.append(explorations)
-        all_contradictions.append(contradictions)
+        all_contradictions.append(contradictions.get()['contradictions'])
     
     x_data = np.arange(10, batch_size * (n_iter - 1) + 11, batch_size)