(9+10+11+12+8)/5 = 10
Let us see how the values obtained differ in respect to 10:
9 10 11 12 8 10
-1, 0, +1, +2, -2 = differences from the mean average figure. We now take the average of these differences, disregarding the plus and minus signs:
(1+0+1+2+2)/5 = 6/5 = 1.2 = mean average error
The personal mean error is a datum that it is necessary to know in order to give value to any measurements that we may wish to give forth.
In taking the various test measurements for the purpose of calculating one's personal error, it is well to use the precaution of not taking them twice at the same sitting, but after an interval of time, not only so that all marks will have disappeared that may have been left upon the skin by the instrument in the act of measuring, but also that the preceding figure will have faded from our memory. Accordingly, the measurements should be repeated on successive days and if possible under the same conditions of time and place.
It is well to make a careful choice of the time and place, because these also have their effect upon the figures.
It will be observed that if the measurements are made in a well-appointed place, with a steady light, without noises, in short, without disturbing causes, the personal error is much more easily decreased, i.e., the measurements are more exact, because the measurer can better concentrate his attention.