CHAPTEE V. 



NORMAL VARIABILITY. 



Schemes of Deviations. — Normal Curve of Distribution. — Comparison of 

 the observed with the Normal Curve. — The value of a single Devia- 

 tion at a known Grade determines a Normal Scheme of Deviations. — 

 Two Measures at two known Grades determine a Normal Scheme 

 of Measures. — The Charms of Statistics. — Mechanical illustration of 

 the Cause of the Curve of Frequency. — Order in apparent Chaos. — 

 Problems in the Law of Error. 



Schemes of Deviations. — We have now seen how easy 

 it is to represent the distribution of any quality among a 

 multitude of men, either by a simple diagram or by a line 

 containing a few figures. In this chapter it will be shown 

 that a considerably briefer description is approximately 

 sufficient. 



Every measure in a Scheme is equal to its Middlemost, 

 or Median value, or M, plus or minus sl certain Devia- 

 tion from M. The Deviation, or "Error" as it is 

 technically called, is plus for all grades above 50°, zero 

 for 50°, and minus for all grades below 50°. Thus if 

 (±D) be the deviation from M in any particular case, 

 every measure in a Scheme may be expressed in the 



E 2 



