The Ishikawa Diagram is a cause-and-effect diagram that helps track down the reasons for imperfections, variations, defects, or failures. The diagram looks just like a fish skeleton (hence Fishbone Diagram), with the problem at its head and the causes for the problem feeding into its spine.
An Ishikawa Diagram should be created once a problem has been detected and potential causes have been collected. It should be viewed as a graphical depiction of hypotheses that could explain the failure under investigation. In other words, the elements in the Ishikawa Diagram should be able to explain how the failure happened. It serves to quickly communicate these hypotheses to team members and management.
For example, a “lighting” problem in a production line could cause an employee to make mistakes. As such, the result of bad lighting should be listed and then empirically investigated using an Ishikawa Diagram. To get to the root cause of a problem or failure, the investigation can be aided by answering the 5 Whys. That means, answering "why?" as many times as necessary to understand why a problem may be occurring.
Here's an example of what the 5 Whys could look like:
Why is the operator failing and making mistakes? Because they cannot keep up with the line and catch defective units
Why can they not keep up with the line and catch defective units? Because they cannot see every unit in detail
Why can't they see every unit in detail? Because visibility is affected
Why is visibility affected? Because the room is too dark
Why is the room too dark? Because the light bulb is broken and hasn't been changed
Thus, the root of the problem is not the operator or their abilities, but rather, that there is insufficient light in the room for them to perform their job most effectively. The solution? The light bulb needs to be changed or more lighting needs to be installed in the area to ensure optimum productivity.
Source: George, M Upton, M Price, M (2004), "The Lean Six-Sigma Pocket Tool Box", Mc Graw-Hill, NY