When there is a problem or issue, we need to get to the root of that problem, otherwise we can’t reach to the solution. Sometimes some specific problems are like a bad weed, it becomes the headache for us if we don’t know the root cause of it but there is a hard time choosing between simple Root Cause Analysis and using the Six Sigma methodology. Although the application of the Six Sigma methodology doesn’t have to be rigid, formal and complicated, it was designed for more complex problems than basic root cause analysis (RCA). So, it is a specific technique that comes along with specific tools that help identify the root causes of issues.

Some of the powerful Root Cause Analysis (RCA) tools are as follows:

Ishikawa Diagram (also known as Fishbone Diagram or Cause and Effect Diagram (CED))
5 Whys
Current Reality Tree (CRT)
8 Disciplines (8D)
Fishbone (Ishikawa) Diagram
The Ishikawa Diagram is called as Fishbone Diagram because the completed pictorial diagram looks very much like the shape of a fishbone. This is a simple and effective tool that shows the problem causes (inputs) for a given effect (output).

The causes are usually grouped into the following categories (also known as 6M):

Men (People) – personnel involved in the process
Methods – methods in which the process is performed
Machines – equipment / tools required for the process
Materials – materials required for the process
Measurements – measurements and data collected
Mother Nature (Environment) – environment in which the process operates
The causes under each category are identifies using brainstorming session. For each causes, we should be asking: “Why is this happening”? Using 5Why? And add reason to the diagram. Repeat the procedure for asking “Why for “other causes as well

5 Whys
5 Whys is a simple but powerful technique for root cause analysis (RCA). The basic idea is to ask “Why? “more than five times to find the real root cause. The reason for making this such a priority is to identify the real problem of an issue to fix the problem in all its permutations once, instead of fixing many different surface problems that the major underlying problem is causing.

Current Reality Tree (CRT)
CRT is a logic-based tool for using cause and effect relationship to determine root problems that cause the undesirable effects in a given system.

8D
The 8 Disciplines (8D) method is a problem-solving approach that is widely used in the automotive and space industries. The objective of 8D is to identify, correct and eliminate recurring problems. 8D provides a framework of 8 stages and other RCA methods (such as Fishbone, 5 Whys, CRT) can use used within the 8D framework without any conflict.

Differentiating between when to apply the Six Sigma Methodology and Root Cause Analysis doesn’t have to be a mystery to anyone. By using these four simple tools for each you can make the right decision.

Reference - http://www.niqcgroup.com/four-root-cause-analysis-tools-in-six-sigma/