diff --git a/README.md b/README.md
index 20cb9ca..dc65c09 100644
--- a/README.md
+++ b/README.md
@@ -48,6 +48,9 @@ composer require php-ai/php-ml
     * [DBSCAN](http://php-ml.readthedocs.io/en/latest/machine-learning/clustering/dbscan)
 * Cross Validation
     * [Random Split](http://php-ml.readthedocs.io/en/latest/machine-learning/cross-validation/random-split)
+* Preprocessing
+    * [Normalization](http://php-ml.readthedocs.io/en/latest/machine-learning/preprocessing/normalization)
+    * [Imputation missing values](http://php-ml.readthedocs.io/en/latest/machine-learning/preprocessing/imputation-missing-values)
 * Feature Extraction
     * [Token Count Vectorizer](http://php-ml.readthedocs.io/en/latest/machine-learning/feature-extraction/token-count-vectorizer)
 * Datasets
@@ -57,6 +60,7 @@ composer require php-ai/php-ml
 * Math
     * [Distance](http://php-ml.readthedocs.io/en/latest/math/distance/)
     * [Matrix](http://php-ml.readthedocs.io/en/latest/math/matrix/)
+    * [Statistic](http://php-ml.readthedocs.io/en/latest/math/statistic/)
     
 
 ## Contribute
diff --git a/docs/index.md b/docs/index.md
index 20cb9ca..dc65c09 100644
--- a/docs/index.md
+++ b/docs/index.md
@@ -48,6 +48,9 @@ composer require php-ai/php-ml
     * [DBSCAN](http://php-ml.readthedocs.io/en/latest/machine-learning/clustering/dbscan)
 * Cross Validation
     * [Random Split](http://php-ml.readthedocs.io/en/latest/machine-learning/cross-validation/random-split)
+* Preprocessing
+    * [Normalization](http://php-ml.readthedocs.io/en/latest/machine-learning/preprocessing/normalization)
+    * [Imputation missing values](http://php-ml.readthedocs.io/en/latest/machine-learning/preprocessing/imputation-missing-values)
 * Feature Extraction
     * [Token Count Vectorizer](http://php-ml.readthedocs.io/en/latest/machine-learning/feature-extraction/token-count-vectorizer)
 * Datasets
@@ -57,6 +60,7 @@ composer require php-ai/php-ml
 * Math
     * [Distance](http://php-ml.readthedocs.io/en/latest/math/distance/)
     * [Matrix](http://php-ml.readthedocs.io/en/latest/math/matrix/)
+    * [Statistic](http://php-ml.readthedocs.io/en/latest/math/statistic/)
     
 
 ## Contribute
diff --git a/docs/machine-learning/preprocessing/imputation-missing-values.md b/docs/machine-learning/preprocessing/imputation-missing-values.md
new file mode 100644
index 0000000..e69de29
diff --git a/docs/machine-learning/preprocessing/normalization.md b/docs/machine-learning/preprocessing/normalization.md
new file mode 100644
index 0000000..e69de29
diff --git a/docs/math/statistic.md b/docs/math/statistic.md
new file mode 100644
index 0000000..89cc00e
--- /dev/null
+++ b/docs/math/statistic.md
@@ -0,0 +1,7 @@
+# Statistic
+
+### Correlation
+
+### Mean
+
+### Standard Deviation