SECTION II— USE OF GRAPHIC METHODS 



PROBLEM 8. (Office.) The Fundamental Principles in the Use of 

 Graphic Methods. 



Explanation. — In this problem one of the simplest problems involving the 

 plotting of curves has been chosen and outlined primarily with reference to 

 illustrating the fundamental principles of determining averages by means of 

 plotted values, and to show something of the significance of bringing out a 

 series of related results by graphic representation. 



Illustration. — To Make a Table of Average Heights on Diameters by Plot- 

 ting the Values on Co-ordinate Paper. 



Directions : 



A. Data Required. — Measurements showing the total heights of trees of dif- 



ferent diameters at breast height. Use Data Series I in the Appendix. 



B. Method of Procedure. 



(a) Preparing the Co-ordinate Paper. 



1. The problem used to illustrate this exercise aims to determine an 



average height for certain specified diameters, i.e., the diameters 

 of the trees are independent variable quantities and the heights 

 dependent variables. As it is customary to let abscissae on the 

 cross-section paper represent the independent values and 

 ordinates the dependent values, in this problem let the abscissa? 

 represent diameters and the ordinates heights. It is a rule 

 to let horizontal distances from the vertical axis represent 

 abscissae, and vertical distances from the longitudinal axis 

 ordinates. 



2. Having determined which values shall be represented by abscissae 



and which by ordinates, the next step is to determine the limits 

 of variation which the data will represent {i.e., in this problem, 

 what are the smallest and the largest values for diameters and 

 for heights that it will be necessary to plot?), so that the unit 

 best adapted for each of the co-ordinate axes can be decided 

 upon. In deciding this unit, remember that the larger the unit 

 the more accurate will be the results (so that the entire sheet 

 of paper should be utilized as far as practicable), and also that 

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