The Five "Whys" is a tool that is commonly used in the cause analysis phase of HPT:

It almost sounds like too simplistic of a tool to be used in the analysis of critical problems, but by asking the question "Why" several times you can often get to the actual or root cause of the problem.  Five is not the rule for the number of times "Why" should be asked; however, the use of this tool over the years in a myriad of business, education and other fields with good results has yielded that asking "Why" five times gets to the root cause.  Once the root cause is uncovered using the five "Whys" tool, the real business of solving the problem can commence.

Benefits of the 5 "Whys" Tool:

• It helps determine the root cause of a problem.
• It can help groups differentiate between symptoms of the problem and the actual problem.
• It is one of the simplest tools to use; extensive training is not required.
• Since it requires no statistical analysis, it can be used to solve Human Resources type problems in an organization.

Example of Using the 5 "Whys" Tool:

Let's take the example of an airline that is receiving a large number of customer complaints about lost baggage.

Problem Statement:  Customers are angry about their luggage being lost.

1. Why are they angry about their luggage being lost?

- Because they are missing their clothing and other items they need with them at their destination. 
  
2. Why are they missing their items at their destination?

- Because it was not placed in the cargo hold of their flight so it will arrive with them. 
  

3. Why was their luggage not placed in the cargo hold of their flight?

- Because it was not placed in the cart that takes the luggage to the cargo hold door.

4. Why was it not placed in the cart that takes the luggage to the cargo hold door?

- Because the luggage did not have a tag indicating the flight it should travel on or it had a tag with incorrect flight information.

5. Why did it not have a tag or have a tag with the incorrect flight information?

- Because the tags are handwritten by ticketing agents and attached to luggage with elastic string.

6. Why are the tags handwritten by ticketing agents and attached with elastic string?

- Because there is no automated system to print out baggage tags and attach to luggage.

7. Why isn't there an automated system to print out baggage tags and attach to luggage?

- It is costly to design and implement a new system.

In this problem the question had to be asked seven times in order to find out the actual issue.  The symptoms of the issue of lost luggage were the wrong tags, missing tags, and luggage not making it to the cart bound for the cargo door hold.  The actual problem was that money was not available to automate the process of matching bags with the correct flight and ensuring the tag stayed on the luggage.

In this case, which actually happened at an airline in the 1990s, the problem of budgeting money for a luggage tag system was addressed.  Money was allocated to the information technology department to create a system to automate luggage tagging, and people were hired or reassigned to create the system.  After the system was tested and implemented was a luggage tag "sticker" that is printed out upon check-in and affixed to luggage.  The end result was a decrease in lost luggage, fewer customer complaints due to the decrease, and happier customers.

References:
Rothwell, W., Hohne, C. & King. S. (2007). Human Performance Improvement (2nd ed.). Boston: Elsevier.