168 



CONSTRUCTION OF STANDARD VOLUME TABLES 



one variable when based upon another definite variable, as for instance, the increase 

 in volume due to increase in diameter by 1-inch classes, it is always preferable to 

 plot the independent variables on the horizontal scale and the dependent variables 

 on the vertical scale. 



Neglect of this precaution not only conveys an ocular impression the reverse of 

 the actual law, but tends to create the false notion that the two variables are inter- 

 changeable, whereas one must always be an independent or fixed base, on which 



60 



.g 3"5 



2ff 



10 11 12 13 14 15 16 17 18 19 



D.B.H. Inches 



Fig. 27. — Curve of volume based on D.B.H. for trees of a single height class. 



the required data are collected, classified and arranged. For instance, in deter- 

 mining the relation between D.B.H. and age of trees, absolutely different results 

 are obtained if in the first instance, the average D.B.H. is found for all trees of 

 given age classes, and in the second, the average age is determined for all trees of 

 given D.B.H. classes (§ 275). The values of these tables or curves are not inter- 

 changeable. The dependent variable can always be identified as the one whose 

 values are sought; the independent, the one whose values are already known. 



The use of curves, or graphic plotting, enables the investigator to obtain a 

 given degree of accuracy with a greatly reduced number of field measurements. 



