THE USE OF CORRECTION FACTORS FOR VOLUME 295 



Since this error consists in applying the per cent erroneously to the 

 area estimated within the strip, it diminishes with the per cent covered 

 by the strip; e.g., should 25 per cent of the above tract be estimated 

 and found to contain 50,000 board feet, and the correction factor be 

 actually 10 per cent, the remaining area, which if uncorrected would 

 have a stand of 150,000 board feet, has actually 10 per cent more than 

 this or 165,000 board feet or a total for the tract, of 215,000 board feet. 

 But applying 10 per cent to the entire tract indicates a total stand of 

 220,000 board feet or an error of +2.4 per cent. But with the decrease 

 in the per cent tallied, the probability of obtaining a close observation 

 of the remainder and applying a correct per cent also diminishes so 

 that if a correction factor is used at all, there is less need for modifying 

 the per cent. The conclusion is that when, on account of measuring 

 a large per cent of the area, it is possible successfully to use a correction 

 factor as applied to the remainder, there is all the greater necessity 

 for making a correct application of this factor. 



To determine the actual correction from a per cent obtained by 

 weighted observations, two methods may be used. The first of these 

 methods applies to irregular areas where the per cent estimated is not 

 uniform, that is, in areas estimated by the separation of types. The 

 steps are as follows: 



1. Reduce the stand on strip to stand per acre. 



2. Apply the per cent correction to this stand per acre. 



3. Calculate the stand separately for the area not estimated, using 

 the corrected average stand. 



4. Add together the estimates on and off the strip for the total; 

 e.g., on 100 acres, 17 per cent is estimated and the remaining 83 acres 

 is judged to run 10 per cent heavier than the strip. The tally on the 

 strip is 170,000 board feet, averaging 10,000 board feet per acre. The 

 10 per cent correction gives 11,000 board feet per acre off the strip, or 

 a total estimate off strip of 913,000 board feet. The total, both on 

 and off strip is 1,083,000 board feet. 



The second procedure may be applied when the per cent estimated 

 is uniform and type or area correction seldom applied. The rule is, 

 reduce the correction -per cent by the proportion which the area estimated 

 in the strip bears to the total area. E.g., where the strips cover one-half 

 the area or 50 per cent, a correction factor of 10 per cent applies to the 

 other 50 per cent or one-half. Then, . 50 X. 10 =.05. A 5 per cent cor- 

 rection can be apphed to the total normal estimate. Where 25 per cent 

 is estimated and a 10 per cent correction is found, this applies only 

 to three-quarters of the area; .75X.10 is .075. The correction factor 

 of 7| per cent may then be applied to the total area. It makes no dif- 

 ference whether a correction of 10 per cent is applied to 75 per cent 



