Which expression correctly defines percent error in an experiment?

• A. (experimental - true) / true times 100
• B. |experimental - true| / true times 100
• C. (experimental - true) / true
• D. |true - experimental| / experimental

Answer: (B)

Percent error tells you how far your measurement is from the true value, expressed as a percentage of the true value. To capture the size of the error regardless of direction, you take the absolute difference between the experimental value and the true value: |experimental value − true value|. Then you divide by the true value to relate the error to what you were actually measuring, and multiply by 100 to turn it into a percent. So the correct form is the absolute difference over the true value, times 100.

If you drop the absolute value, you could get negative percentages when you underestimate, which isn’t useful for showing how big the error is. If you divide by the experimental (measured) value instead of the true value, the result depends on your measurement and doesn’t reflect the true quantity you’re aiming to represent.