COLLECTION AND CALCULATION OP STATISTICAL DATA. 55 



(a) Preparation of Yield Tables, 



The ordinary method employed in India is the usual modifica- 

 tion of Baur's method, whereby for each species, large numbers of 

 evenaged and fully stocked sample plots of all ages and all qualities 

 are laid out, thinned according to a definite standard, and 

 measured periodically. Each sample plot at each measurement 

 should supply the following data : 



Average age, height, diameter, and volume per tree. 



Total (per acre) basal area, and volume of main crop and of 

 thinnings. 



In order to obtain this information accurately the following 

 measurements are made as accurately as possible : 



(1) Area of sample plot. 



(2) Mean diameter of each tree in the plot (they are serially 



numbered). 



(3) Mean diameter of each tree removed in thinnings. 



(4) Height, diameter, form factor, and volume of a number 



of sample trees, from which the volumes, etc., of the 

 crop and of the thinnings are calculated. Sample trees 

 are used only for obtaining height, form factor, and volume 

 curves, never for direct calculation of corresponding 

 figures for the plot. Howard's note on the " Collection 

 and calculation of statistical data " published in 1921 

 gives such a detailed and complete description, both 

 of the field work and of the calculations, that no 

 further description is required here. But to give some 

 idea of the amount of research work required before a 

 satisfactory yield table can be compiled from the data 

 collected, it may be said that at least 25 plots (most of 

 which have had at least one remeasurement), should be 

 available for each quality class, and no satisfactory and 

 comprehensive yield table can be prepared unless each 



