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PSYCHOLOGY: J. A. HARRIS 
By personal equation we understand a definite bias in a given direc- 
tion. In a series of estimates by an observer the errors in excess of the 
true value may be no more numerous and no greater in amount than 
those in defect. The average of the deviation of the estimates from the 
true number of objects will then be 0 plus or minus a small amount due 
to the errors of random sampling. Such an observer may be said to 
have no personal equation. Other individuals, however, may have a 
definite tendency to err on one side of verity in their evaluations. Such 
may be said to have a positive or a negative personal equation, as the 
case may be. Personal equation is measured by the mean, regarding 
signs, of the deviation of the samples from their ideal value. 
But quite without reference to personal equation, one observer may be 
more erratic than another, estimating now far too high, now far too 
low. By steadiness of judgment we mean consistency in estimation as 
measured by the closeness with which the errors of estimation cluster 
around their mean value. Steadiness of judgment may be expressed 
in the absolute terms of the standard deviation of the errors of esti- 
mation about their mean (S.D.), or in the relative terms of the coefficient 
of variation (C.V.). 
where 2 is the conventional summation sign, N is the number of esti- 
mates and d indicates the deviation of the estimate from the true number 
of objects, i. e., the actual number laid out less the required number; and 
where M is the constant number which the observer seeks to lay out 
plus or minus the observed personal equation, as the sign of the latter 
may indicate. 
In much of the work in which personal equation is a factor the ob- 
server is not able to check his estimates against the true values, and so 
attempt at each successive observation to profit by his previous experi- 
ence. In these experiments each observer made a persistent effort to 
improve. This was based on a knowledge of the immediately preced- 
ing errors, and consisted in a constant effort to lay out exactly the de- 
sired number of seeds. Thus the influence of experience upon both 
Total estimates 
which here is most easily calculated from the formula 
c.v, = ms.D./M, 
