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 a 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 



