16 STATISTICAL METHODS. 



CHAPTER III. 



THE CLASSES OF FREQUENCY POLYGONS. 



The plotted curve may fall into one of the folio wing classes : 



A. Unimodal. 



I. Simple. 



1. Range unlimited in both directions: 



a. Symmetrical. The normal curve. 

 It. Unsymmetrical (Pearson's Type IV). 



2. Range limited in one direction, together with 



skewness (Type III). 



3. Range limited in both directions : 



a. Symmetrical, Type II. 



b. Unsymmetrical, Type I. 

 II. Complex. 



B. Multi modal. 



The classification of any given curve is not always an easy 

 task. Whether the curve is unimodal or multimodal can be 

 told by inspection. "Whether any unimodal curve is simple 

 or complex cannot be told by any existing methods without 

 great labor and uncertainty in the result. 



Complex curves may be classified as follows : 



1. Composed of two curves, whose modes are different but so near that 

 the component curves blend into one ; such curves are usually unsym- 

 metrical. 



2. The sum of two curves having the same mode but differing varia- 

 bility. 



3. The difference of two curves having the same mode but differing 

 variability. 



If the material is believed to be homogeneous and the curve 

 is unimodal it is probably simple and its classification may be 

 carried further. 



For classification the rule is as follows : Determine the mean 

 of the magnitudes. Take a class near the mean (call it V m ) 



