The Ishikawa diagram, also known as a fishbone diagram or cause-and-effect diagram, is the best tool to determine the root cause of variations in a process. This diagram helps teams visually map out all potential causes of a problem, categorizing them into major factors such as methods,machinery, materials, and people. By exploring these potential causes systematically, teams can identify the root causes of variations and focus their improvement efforts accordingly.

Histogram (A): A histogram is used to display the distribution of data points but does not help in identifying root causes.

Shewhart chart (C): Also known as a control chart, it monitors process stability over time but is not specifically for root cause analysis.

Scatter plot (D): A scatter plot shows relationships between two variables but does not identify root causes of variations.

References

NAHQ Body of Knowledge: Root Cause Analysis Tools in Quality Improvement

NAHQ CPHQ Exam Preparation Materials: Using Ishikawa Diagrams for Root Cause Analysis

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