inventory purposes. Three sample-plot sizes were used to measure growth: 

 1/500 acre for trees 5.0 inches to 10.9 inches d.b.h., l/50 acre for 

 trees 11.0 inches to 24.9 inches d.b.h. and l/5 acre for trees 25.0 

 inches d.b.h. and larger. In Idahc County the maximum for the l/50 

 acre was 30.9 inches d.b.h. and the l/5-acre plot included all trees 

 31.0 inches d.b.h. and larger. Radial growth was determined for the 

 10-year period preceding the date of field examination. Radial growth 

 was converted to volume growth by subtracting the reconstructed volume 

 of sample trees 10 3^ears ago, taking into account changes in height 

 and bark thickness as well as diameter, from current sample tree vol- 

 ume. For the total growth determination sample trees were divided into 

 two classes: (1) ingrowth trees, those that either developed from sap- 

 ling into pole size or that grew out of pole size into sawtimber size, 

 and (2) main-growth trees, those that remained either of pole size or 

 of sawtimber size during the growth-study decade. Main-growth percents 

 were calculated and applied to total inventory volumes by stand-size, 

 species, and diameter-class groups. The gross growth was modified by 

 reductions for cull and mortality, and by additions for ingrowth and 

 growth on cut trees. 



Mortalit y 



Mortality was based on a tally of trees judged to have died during the 

 last half of the growth-study period (5 years) on each l/5-acre plot 

 sampled for inventory. The five-year mortality was doubled to obtain 

 estimated mortality for the entire growth-study period. 



Commodity Dra in 



The statistics on commodity drain are based chiefly on the reports of 

 producing or consuming industries, xvith adjustments for waste as de- 

 termined from woods and mill-scale studies. 



ACCURACY OF ESTIMATES 



In determining the area, volume, and growth of the various cover types 

 and species there are two possible sources of errors (l) human errors 

 in classifying the cover of field samples, in measurements on sample 

 plots, and in compiling the field data, and bias resulting from the 

 improper construction, selection, and use of tree-volume tables; (2) 

 sampling errors. The former result from mistakes of judgment or tech- 

 nique and from the complexity of the cover which often grades from 

 one class into another with no clearly defined boundaries. It is 

 seldom possible to evaluate such errors. An effort was made to main- 

 tain a high order of accuracy and uniformity of standards in the classi- 

 fication, collection, and compilation of sample data by field checks, 

 by a continuing program of training, and by cross-checks in the office. 



Sampling errors (standard errors), on the other hand, do not involve 

 human errors, but rather are theoretical measures of the reliability 

 of estimates based on the variability of sample measurements. For a 

 given condition they vary inversely with the square root of the number 



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