Personal equation

[1] The term originated in astronomy, when it was discovered that numerous observers making simultaneous observations would record slightly different values (for example, in recording the exact time at which a star crossed the wires of a reticule in a telescope), some of which were of a significant enough difference to afford for problems in larger calculations.

[3] The problem was forgotten and only analysed two decades later by Friedrich Wilhelm Bessel at Königsberg Observatory in Prussia.

Setting up an experiment to compare the values, Bessel and an assistant measured the times at which several stars crossed the wires of a reticule in different nights.

In response to this realization, astronomers became increasingly suspicious of the results of other astronomers and their own assistants and began systematic programs to attempt to find ways to remove or lessen the effects.

[6] He continued to wrestle in later writings with the problems of psychological solipsism and infinite regress this potentially posed,[7] and considered every therapist should have at least a good working knowledge of his or her own personal equation.