True I’m a mariner been one my whole life ocean land mountain arctic celestial

The terms variation and deviation refer to how a measured or observed value differs from a true baseline. While they share similar meanings, they apply to distinct contexts across navigation, statistics, and science:[1, 2]

Navigation and Aviation

In flying and sailing, both terms describe errors that cause a magnetic compass to point away from True North. [1, 2]

• Variation (Magnetic Declination): The difference between True North (Earth’s geographic pole) and Magnetic North. This error is caused by the Earth itself and depends strictly on your geographic location. [1, 2, 3, 4]
• Deviation: The error caused by the vessel or aircraft’s own metallic body and electrical equipment. This error is unique to each specific plane or boat and changes depending on the direction you are steering.[1, 2, 3, 4]

For a clear visual breakdown of how these magnetic errors interact and how pilots and sailors correct for them:

Mathematics and Statistics

In statistics, the two concepts describe how data points differ from a central value. [1, 2, 3]

• Variation (Variance): A measure of how far a set of numbers is spread out from their average value.
• Deviation: The difference between a single, specific data point and the mean (or expected value) of the dataset. [1, 2, 3, 4]

Science and Engineering

• Variation: Generally refers to a natural, expected fluctuation or change in a process over time (e.g., genetic variation, temperature variation). [1, 2, 3]
• Deviation: Refers to a specific discrepancy or intentional alteration from a standard or rule (e.g., standard deviation, a deviation from a flight path). [1, 2, 3, 4]

Project Management

• Variation: Natural fluctuations within an established process or system.
• Deviation: An unplanned event or a distinct change from a baseline, standard, or project plan that requires a corrective action.