Sampling errors (standard errors of estimate) on the other hand do 

 not involve human errors but rather are theoretical measures of the 

 reliability of estimates based on the variability exhibited by 

 sample measurements. They generally vary inversely with the square 

 root of the number of samples and directly with the square root of 

 the unsampled proportion of the total population. Hence, they can 

 be controlled by altering either the number of samples, the size of 

 individual samples, or both. 



Analysis of sample variations indicate that the standard errors of 

 estimate for the unit as a whole are - 3.5 percent for total forest, 

 i 5.1 percent for commercial forest, and - 6.6 percent for noncom- 

 mercial forest. Accordingly, the probabilities are 2 out 3 that the 

 actual forest, commercial forest, and noncommercial forest land areas 

 are, respectively, within ± 85,000, - 90,000, and ± 44,000 acres of 

 the estimated areas if measurements and computed errors introduced no 

 bias . 



In determining timber volumes, the possible sources of error include 

 in addition to those cited above (3) inaccurate measurement of sample 

 plots, tree diameters, tree heights, and cull, and (4) bias resulting 

 from improper construction, selection, and use of tree-volume tables. 

 All reasonable effort was made to eliminate errors from these sources. 

 The standard error of the board-foot volume estimate for the block as 

 a whole is - 7.8 percent and of the cubic-foot volume estimate, - 6.8 

 percent. Accordingly, the probabilities are 2 out of 3 that the 

 actual volumes are within ± 324,000 M board feet and - 104,000 M 

 cubic feet of the given estimates. 



The reliability of one statistic as compared with another presented 

 in the same or a related table can be judged roughly by its relative 

 magnitude. In general, the larger quantities warrant greater con- 

 fidence; the smaller quantities indicate only relative magnitude. 

 This fact should be borne in mind in considering the small quantities 

 associated with many of the counties covered in this report. 



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