was calculated from simple growth percents (as 
determined from the relationship of sample-tree 
volume and sample-tree growth by type, stand-size, 
stocking, species, and diameter-class groups) and 
total inventory volumes by the same stratifications. 
Net growth was calculated by subtracting mortality 
from gross growth. 
Commodity Drain 
Commodity drain for both sampled and com- 
plete survey areas is based chiefly on the reports 
of producing or consuming agencies, with adjust- 
ments for waste as determined from woods and 
mill-scale studies. Approximately 88 percent of 
the board-foot drain estimates were derived from 
a complete canvass of sawmills, pulp mills, pole 
manufacturers, railroads, and mines. The remain- 
der, consisting chiefly of drain for posts and fuel 
wood, was derived in part from production and 
sales reports, and in part from consumption ratios, 
i. e., fence posts used by size of farm. 
Mortality 
Mortality in “growing stands” of the area cov- 
ered by complete survey was based on a tally of 
trees judged to have died within the 10-year period 
of the growth study. Mortality of “nongrowing 
stands” was derived by applying the mortality 
percent of “growing stands” to the volume of “‘non- 
growing stands.” 
Mortality for the sample survey was based on 
a tally of trees judged to have died during the 
last half of the growth-study period (5 years) on 
each 1/5-acre plot sampled for inventory. The 
5-year mortality was doubled to obtain mortality 
for the entire growth-study period. 
Accuracy of Estimates 
Area 
In determining the extent of various cover types 
and stand-size classes, there are two possible sources 
of error—errors in classifying the cover of the field 
samples and in compiling the field data; and 
sampling errors. The former result from mistakes 
of judgment or technique 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 maintain a high order of ac- 
curacy and uniformity of standards in the classi- 
fication, collection, and compilation of sample data 
by field checks, by a continuing program of train- 
ing, 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 esti- 
mates based on the variability of sample measure- 
ments. ‘hey generally vary inversely with the 
square root of the number of samples and directly 
with the square root of the unsampled part of the 
total population. Hence, they can be controlled by 
altering either the number of samples, the size 
of individual samples, or both. 
Area estimates for the part covered by complete 
survey have no sampling errors. For the part cov- 
ered by sampling, the probabilities are 2 out of 
3 that sampling errors are within the following 
limits: 
Estimated 
area Standard error 
(acres) (acres) (percent) 
Total=forest-land= 4-5 7,321,000 +146,000 +2.0 
Commercial forest land...... 5.874.000 +152,000 +2.6 
Noncommercial and reserved 
forestland meses 1,447,000 + 63,000 +44 
For the State as a whole, considering sampling 
and technique errors for the sampled area and 
technique errors for the area covered by complete 
survey, it is judged that actual areas lie between 
the following limits: 
Million acres 
KLotal=forest) land <sac ae ee ee 22.3-22.5 
Gommercialzforest:. landa= aie a ee 15.7-15.9 
6.5— 6.7 
Volume 
In determining timber volume, the possible 
sources of error, in addition to those cited under 
“Area” and “Growth,” include: inaccurate measure- 
ment of sample plots, tree diameters, tree heights, 
cull, and bias resulting from the improper construc- 
tion, selection, and use of tree-volume tables. All 
reasonable effort was made to eliminate errors from 
these sources. 
Sampling errors for the area covered by sampling 
are =-3.2 percent or +212 million cubic feet for 
the primary growing stock and +9.0 percent or 
--1,397 billion board feet for the live saw-timber 
estimate. 
Comparison of survey volumes and cutting 
records indicates that the volume estimates for 
the area covered by complete survey are slightly 
58 Forest Resource Report No. 5 U. S. Department of Agriculture 
