. Handbook of mental examination methods. alculated and the average obtained we find there is an average error of mm. More thanthis, the reproductions are sometimes larger and sometimessmaller than the original. If the corresponding errors are giventheir appropriate signs, plus or minus, the algebraic sum can befound and the average determined. We find the so-called con-stant error in this manner. In this case and in all similar cases,the constant error is the magnitude minus the average, whichfigure always retains its proper algebraic sign, plus or minus. We may say, therefore, that in dr
. Handbook of mental examination methods. alculated and the average obtained we find there is an average error of mm. More thanthis, the reproductions are sometimes larger and sometimessmaller than the original. If the corresponding errors are giventheir appropriate signs, plus or minus, the algebraic sum can befound and the average determined. We find the so-called con-stant error in this manner. In this case and in all similar cases,the constant error is the magnitude minus the average, whichfigure always retains its proper algebraic sign, plus or minus. We may say, therefore, that in drawing a line of 100 mm. thesubject tends to get the reproductions too large (there is a plusconstant error) and that the average differs only by 1 per cent. OBSERVATIONAL DATA 181 from the original, but that the variation in the performance per cent. The facts of the variation in measurements are made plain ifthey are indicated by a graph, or curve. This is constructedwith a base line, showing the thickness, or lengths, or weights,. B A B Fig. 40. Distribution of observations in experiment on the ability toreproduce a given length of line. or other measures and with a horizontal line or area represent-ing the number of measurements of the particular amount. The results of the test of drawing a line of 100 mm. length arerepresented graphically in Fig. 40. If the number of observa-tions had been much larger the curve of the distribution of theobservations would have been smoother and have the form shownin Fig. 41. This is known as the probability curve. On thesecurves the average and the average variations are shown. It
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