56 



MISSISSIPPI'S FOREST RESOURCES AND INDUSTRIES 



are sound, reasonably straight, and •without large knots 

 or knot clusters; they are suitable for ties and low- 

 grade structural timbers. 



Stand Quality 



Fair and better. — A stand in which grade 2 or 

 better hardwood logs or softwood trees are present. 



Poor. — A stand in which no grade 2 hardwood logs 

 or softwood trees are present. 



The Field Survey and Its Accuracy 



Forest Land Area and Timber J'olume 



An average of 14 years elapsed between the two 

 Forest Surveys of Mississippi, though the interval 

 varied for the different regions of the State. 



The 1946-48 data on forest acreage and timber vol- 

 umes are based on a forest-nonforest dot classification 

 made on aerial photographs and on ground classifica- 

 tion and measurement of quarter-acre sample plots 

 located in a square pattern over the State. In 55 

 counties, pairs of plots were located every 3 miles on 

 lines 3 miles apart; in the other 27 counties, the spac- 

 ing was 4.2 miles. 



The accuracy of the Forest Survey estimates may be 

 affected by two types of error. The first type stems 

 from use of a sample to estimate the whole and from 

 variability of the item being sampled. This type is 

 termed sampling error; it is susceptible to a mathema- 

 tical evaluation of the probability of error. The sec- 

 ond type of error derives from human mistakes in 

 measurement, judgment, arithmetic, or recording, and 

 from limitations of method or equipment. 



Methods for calculating the effect of this second type 

 of error — often referred to as reporting and estimating 

 error — have not been developed. Reporting and esti- 

 mating error can, however, be held to a minimum by 

 training, good supervision, checking, and emphasis on 

 careful work. This the Forest Survey constantly 

 attempts to do. 



Statistical analysis of the data, using random-sam- 

 pling formulas, indicates a sampling error of 0.5 per- 

 cent for the State estimate of total forest acreage, 2.1 

 percent for total growing stock, and 2.6 percent for 

 sawlog growing stock. Because the volume estimates 

 are derived from a systematic sample, and tests indicate 

 that systematic sampling is more generally efficient 

 than a random sample of the same size, the estimates 

 of volume sampling error may be considered as setting 

 an upper limit of error, rather than as expressing the 

 actual probability of error. 



As the State total acreage and volumes are broken 

 down by Survey region, county, forest type, species, and 

 other subdivisions, the possibility of error increases and 

 is greatest for the smallest items. The order of this 

 increase is suggested in table 5. 



Table 5. — Approximate sampling error to which Survey estimates of 

 Mississippi's commercial forest acreage, total growing stock, and 

 sawlog growing stock may be liable i 



Commercial forest 



Growin 



g stock 



Sawlog growing 

 stock 



Area 



Sampling 

 error 



Total 



Sampling 

 error 



Total 



Sampling 

 error 



Thousand 





Million 





Million 





acres 



Percent 



cubic feet 



Percent 



hoard feet 



Percent 



16,500 



0.5 



7,500 



2.1 



29,000 



2.6 



10.000 



.6 



5,000 



2.6 



20,000 



3.2 



5,000 



.9 



2,500 



3.7 



10,000 



4.4 



2,000 



1.4 



1,000 



5.8 



4,000 



7.0 



500 



2.9 



300 



10.6 



1,000 



14.1 



50 



9.1 



30 



33.6 



100 



44.5 



1 Liable on a probability of 2 chances out of 3. The figures were obtained 

 by averaging out the areas with 3-mile and those with 4.2-mile plot spacing, 

 and by averaging out the differences in forest variability over the State. They 

 may therefore apply only approximately to any breakdown of a State total. 



In computing the changes that took place between 

 1932-35 and 1946-48, the data from the first Forest 

 Survey were adjusted to make them as closely com- 

 parable as possible to data from the second Survey. 

 This was necessary because of certain basic differences 

 between the two sets of data. For example, published 

 estimates from the first Survey were based on a then 

 offiicial figure of 29,671,680 acres of total land area in 

 Mississippi. This has since been superseded bv a 

 revised figure of 30,348,800 acres (from the Bureau of 

 Census areas of the united states^ 194o), based 

 on more accurate measurement. Thus, while actual 

 land acreage has changed little if at all, the estimate of 

 land acreage has increased 2.3 percent, affecting forest- 

 area and timber-volume estimates in like proportion. 



Again, the boundaries of some of the Survey regions 

 were changed between the two Surveys, so as to put all 

 boundaries on county lines for convenience in compila- 

 tion. Yet again, the lower diameter limit for hardwood 

 sawlog trees was dropped from 13.0 inches d. b. h. on 

 the first Survey to 11.0 inches on the second Survey, 

 in line with changing utilization practices. In addi- 

 tion to these major points, there were a number of 

 procedural differences between the two Surveys that 

 were taken into account. In every case, the data from 

 the first Survey were adjusted to conform to the 

 standards of the second Sur\cy before change was 

 computed. 



