The closeness of the agreement between the result of the measurement and the conventional true value of the quantity. Accuracy should not be confused with precision. (ISO)

Core measurements have well-defined calibration techniques and standards. Logging measurements are characterized during tool design and construction, and calibrated regularly to some standard. The quoted accuracy of a log then depends on the initial characterization, the reproducibility of the standard, and the stability of the measurement between calibrations and under downhole conditions. The actual accuracy also depends on the equipment performing and being operated to specification.