diff --git a/csharp_fundamentals/8_oop.md b/csharp_fundamentals/8_oop.md
index 8ea1397..45ded09 100644
--- a/csharp_fundamentals/8_oop.md
+++ b/csharp_fundamentals/8_oop.md
@@ -174,4 +174,11 @@ Reference:
 * [Usage and Importance of Using in C# -> Using Statment (for garbage collection)](https://www.c-sharpcorner.com/UploadFile/manas1/usage-and-importance-of-using-in-C-Sharp472/)
 
 * .NET has garbage collector to recycle memory. We also has some resource that want to release once used as soon as possible. .NET provide `IDisposable` interface to notify and automatically recycle resource.
-* `using` statement 
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+* `using` statement 
+
+## A Statistical Challenge & Refactoring Statistics
+
+* Copy & Paste multiple lines of code is not good. Stick with DRY
+* Important to separate signing from doing
+
+Refactoring code require **low coupling, high cohesion**
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