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 un- 

 sampled proportion of the total population. Hence, they can be con- 

 trolled by altering either the number of samples, the size of 

 individual samples, or both. 



Analysis of sample variations in the unit indicate that the standard 

 errors of estimate for the unit are ± 1.0 percent for total forest 

 land area and ±3.1 percent for commercial forest land area. 

 Accordingly, the probabilities are 2 out of 3 that the actual forest 

 land area and commercial forest land area are, respectively, within 

 - 10,000 acres and - 19,000 acres of the estimated areas if measure- 

 ments and computing errors introduced no bias. 



In determining timber volumes, the possible sources of error in- 

 clude 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 - 16.2 percent and of 

 the cubic-foot volume estimate, - 8.6 percent. Accordingly, the 

 probabilities are 2 out of 3 that the actual volumes are within 

 ± 97,037 M board feet and ± 24,760 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 rela- 

 tive magnitude. In general, the larger quantities warrant greater 

 confidence; 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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