personal equation

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Psychology Those characteristics of a person that cause variation in observation, judgment, and reasoning.
2. n. Psychology An allowance or adjustment made for such variation.

GNU Webster's 1913

1. n. (Astron.) the difference between an observed result and the true qualities or peculiarities in the observer; particularly the difference, in an average of a large number of observation, between the instant when an observer notes a phenomenon, as the transit of a star, and the assumed instant of its actual occurrence; or, relatively, the difference between these instants as noted by two observers. It is usually only a fraction of a second; -- sometimes applied loosely to differences of judgment or method occasioned by temperamental qualities of individuals.
2. adj. (Astron.) See under Equation.

WordNet 3.0

1. n. variability attributable to individual differences