The difference between the measured value of a quantity and a value adopted as representing the ideal or true value of that quantity. Errors of observation are composed of either one or both of two general classes of error: accidental errors and systematic errors. Constant errors are sometimes placed in a third class but are more often included among the systematic errors as they should be. Errors of observation are also classified according to their origin as external errors, instrumental errors, and personal errors. The algebraic sign of the error of observation is determined from the equation: error of observation = measured value adopted (ideal) value. The term error of observation is dropping out of use and is being replaced by measurement error.