diff --git a/ignite/metrics/recsys/ndcg.py b/ignite/metrics/recsys/ndcg.py
index e2ebd417c8e..0c15990e948 100644
--- a/ignite/metrics/recsys/ndcg.py
+++ b/ignite/metrics/recsys/ndcg.py
@@ -8,12 +8,88 @@
 __all__ = ["NDCG"]
 
 
+class NDCG(Metric):
+    """Computes ndcg
+    `Normalized DCG(DCG) <https://en.wikipedia.org/wiki/Discounted_cumulative_gain>`_.
+
+    .. math::
+        \text{nDCG}_\text{p} = \frac{\text{DCG}_p}{\text{nDCG}_p}
+
+    where :math: \text{DCG}_\text{p} = \sum_{i = 1}^p \frac{2^{rel_i} - 1}{\log_2{(i + 1)}}
+            :math: \text{IDCG}_\text{p} = \sum_{i = 1}^{|REL_p|} \frac{2^{rel_i} - 1}{\log_2{(i + 1)}}
+            :math: \text{$rel_i \in \{0, 1\}$ : graded relevance of the result at position $i$}
+
+
+    - ``update`` must receive output of the form ``(y_pred, y)``.
+
+
+    Args:
+
+        output_transform: A callable that is used to transform the Engine’s
+            process_function’s output into the form expected by the metric.
+        device: specifies which device updates are accumulated on.
+            Setting the metric’s device to be the same as your update arguments ensures
+            the update method is non-blocking. By default, CPU.
+        k: Only consider the highest k scores in the ranking. If None, use all outputs.
+        log_base: Base of logarithm used in  computation
+        exponential: If True, computes exponential gain
+        ignore_ties: Assume that there are no ties in y_score (which is likely to be the case if y_score is continuous) for efficiency gains.
+
+    Examples:
+
+    """
+
+    def __init__(
+        self,
+        output_transform: Callable = lambda x: x,
+        device: Union[str, torch.device] = torch.device("cpu"),
+        k: Optional[int] = None,
+        log_base: Union[int, float] = 2,
+        exponential: bool = False,
+        ignore_ties: bool = False,
+    ):
+        if log_base == 1 or log_base <= 0:
+            raise ValueError(f"Argument log_base should positive and not equal one,but got {log_base}")
+        self.log_base = log_base
+        self.k = k
+        self.exponential = exponential
+        self.ignore_ties = ignore_ties
+        super(NDCG, self).__init__(output_transform=output_transform, device=device)
+
+    @reinit__is_reduced
+    def reset(self) -> None:
+        self.num_examples = 0
+        self.ndcg = torch.tensor(0.0, device=self._device)
+
+    @reinit__is_reduced
+    def update(self, output: Sequence[torch.Tensor]) -> None:
+        y_pred, y_true = output[0].detach(), output[1].detach()
+
+        y_pred = y_pred.to(torch.float32).to(self._device)
+        y_true = y_true.to(torch.float32).to(self._device)
+
+        if self.exponential:
+            y_true = 2**y_true - 1
+
+        gain = _ndcg_sample_scores(y_pred, y_true, k=self.k, log_base=self.log_base, ignore_ties=self.ignore_ties)
+        self.ndcg += torch.sum(gain)
+        self.num_examples += y_pred.shape[0]
+
+    @sync_all_reduce("ndcg", "num_examples")
+    def compute(self) -> float:
+        if self.num_examples == 0:
+            raise NotComputableError("NGCD must have at least one example before it can be computed.")
+
+        return (self.ndcg / self.num_examples).item()
+
+
 def _tie_averaged_dcg(
     y_pred: torch.Tensor,
     y_true: torch.Tensor,
     discount_cumsum: torch.Tensor,
     device: Union[str, torch.device] = torch.device("cpu"),
 ) -> torch.Tensor:
+
     _, inv, counts = torch.unique(-y_pred, return_inverse=True, return_counts=True)
     ranked = torch.zeros(counts.shape[0]).to(device)
     ranked.index_put_([inv], y_true, accumulate=True)
@@ -69,48 +145,3 @@ def _ndcg_sample_scores(
     all_relevant = normalizing_gain != 0
     normalized_gain = gain[all_relevant] / normalizing_gain[all_relevant]
     return normalized_gain
-
-
-class NDCG(Metric):
-    def __init__(
-        self,
-        output_transform: Callable = lambda x: x,
-        device: Union[str, torch.device] = torch.device("cpu"),
-        k: Optional[int] = None,
-        log_base: Union[int, float] = 2,
-        exponential: bool = False,
-        ignore_ties: bool = False,
-    ):
-        if log_base == 1 or log_base <= 0:
-            raise ValueError(f"Argument log_base should positive and not equal one,but got {log_base}")
-        self.log_base = log_base
-        self.k = k
-        self.exponential = exponential
-        self.ignore_ties = ignore_ties
-        super(NDCG, self).__init__(output_transform=output_transform, device=device)
-
-    @reinit__is_reduced
-    def reset(self) -> None:
-        self.num_examples = 0
-        self.ndcg = torch.tensor(0.0, device=self._device)
-
-    @reinit__is_reduced
-    def update(self, output: Sequence[torch.Tensor]) -> None:
-        y_pred, y_true = output[0].detach(), output[1].detach()
-
-        y_pred = y_pred.to(torch.float32).to(self._device)
-        y_true = y_true.to(torch.float32).to(self._device)
-
-        if self.exponential:
-            y_true = 2**y_true - 1
-
-        gain = _ndcg_sample_scores(y_pred, y_true, k=self.k, log_base=self.log_base, ignore_ties=self.ignore_ties)
-        self.ndcg += torch.sum(gain)
-        self.num_examples += y_pred.shape[0]
-
-    @sync_all_reduce("ndcg", "num_examples")
-    def compute(self) -> float:
-        if self.num_examples == 0:
-            raise NotComputableError("NGCD must have at least one example before it can be computed.")
-
-        return (self.ndcg / self.num_examples).item()