. Definition and use of the phi grade scale. Particle size determination; Soils. 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 2 1 1 1 1 1 mm 1 1 1 1 1 -4 Figure 3. The normal curve (for y = , a = ). Shaded area for interval ± la from mean (y) contains the central 68 percent of the area under the curve (adapted from Krumbein, 1957). following estimate of the mean which is statistically more efficient and less biased than the median for cases where the actual gsd is not completely symmetrical. M^ = )16 (4) For a symmetrical distribution, equation (4) will produce the same value as the
. Definition and use of the phi grade scale. Particle size determination; Soils. 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 2 1 1 1 1 1 mm 1 1 1 1 1 -4 Figure 3. The normal curve (for y = , a = ). Shaded area for interval ± la from mean (y) contains the central 68 percent of the area under the curve (adapted from Krumbein, 1957). following estimate of the mean which is statistically more efficient and less biased than the median for cases where the actual gsd is not completely symmetrical. M^ = )16 (4) For a symmetrical distribution, equation (4) will produce the same value as the median. Sx and M^x (eqs. 3 and 4) are probably the best esti- mates of a and y (Inman, 1952) for describing unimodal sedimentary grain-size distributions. A common way to obtain these parameters is by using a graphical technique (Fig. 4). The sample size data are plotted as a cumulative distribution on log (phi) probability paper. This paper is constructed so that a lognormal distribution will plot as a straight line. The plots of sample distributions that are asymmetric will not be straight. The degree of asymmetry, or nonnormality, can be determined by comparing the observed distribution with a straight "approximation" curve drawn 13. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Hobson, R. D. (Richard D. ); Coastal Engineering Research Center (U. S. ). Fort Belvoir, Va. : U. S. Army, Corps of Engineers, Coastal Engineering Research Center ; Springfield, Va. : National Technical Information Service, Operations Division [distributor]
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