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LBEPDFName:
with and denoting the shape parameters of the underlying beta distribution, c and d denoting the lower and upper limits of the log beta distribution, BETPDF denoting the beta probability density function, and where
The log beta distribution has been proposed as an alternative to the log normal distribution. It has the advantage of being able to model both left and right skewness (the lognormal can only model right skewness). It may also be more appropriate when the data has an upper bound. The log beta distribution can be generalized with location and scale parameters in the usual way.
<SUBSET/EXCEPT/FOR qualification> where <x> is a number, parameter, or variable; <alpha> is a number, parameter, or variable that specifies the first shape parameter; <beta> is a number, parameter, or variable that specifies the second shape parameter; <c> is a number, parameter, or variable that specifies the third shape parameter; <d> is a number, parameter, or variable that specifies the fourth shape parameter; <loc> is a number, parameter, or variable that specifies the optional location parameter; <scale> is a number, parameter, or variable that specifies the optional scale parameter; <y> is a variable or a parameter (depending on what <x> is) where the computed log beta pdf value is stored; and where the <SUBSET/EXCEPT/FOR qualification> is optional. The location and scale parameters are optional (the default values are zero and one, respectively).
LET Y = LBEPDF(X,ALPHA,BETA,C,D) PLOT LBEPDF(X,6,6,1,3) FOR X = 1.01 0.01 2.99
LET BETA = <value> LET C = <value> LET D = <value> LET Y = LOG BETA RANDOM NUMBERS FOR I = 1 1 N LOG BETA PROBABILITY PLOT Y LOG BETA PROBABILITY PLOT Y2 X2 LOG BETA PROBABILITY PLOT Y3 XLOW XHIGH LOG BETA KOLMOGOROV SMIRNOV GOODNESS OF FIT Y LOG BETA CHI-SQUARE GOODNESS OF FIT Y2 X2 LOG BETA CHI-SQUARE GOODNESS OF FIT Y3 XLOW XHIGH The following commands can be used to estimate the alpha and beta shape parameters (the lower and upper limit parameters c and d are assumed known) for the log beta distribution:
LET D = <value> LET ALPHA1 = <value> LET ALPHA2 = <value> LET BETA1 = <value> LET BETA2 = <value> LOG BETA PPCC PLOT Y LOG BETA PPCC PLOT Y2 X2 LOG BETA PPCC PLOT Y3 XLOW XHIGH LOG BETA KS PLOT Y LOG BETA KS PLOT Y2 X2 LOG BETA KS PLOT Y3 XLOW XHIGH The default values for ALPHA1 and ALPHA2 are 0.5 and 10. The default values for BETA1 and BETA2 are 0.5 and 10. Note that the log beta percent point function is expensive to compute. For larger data samples, this can make the above fit commands slow. We can do the following to improve the speed of these commands.
title displacement 2 y1label displacement 17 x1label displacement 12 case asis title case asis label case asis y1label Probability Density x1label X . let c = 1 let d = 3 . multiplot corner coordinates 0 0 100 95 multiplot scale factor 2 multiplot 2 2 . title Alpha = 3, Beta = 3 plot lbepdf(x,3,3,c,d) for x = 1.01 0.01 2.99 . title Alpha = 5, Beta = 2 plot lbepdf(x,5,2,c,d) for x = 1.01 0.01 2.99 . title Alpha = 2, Beta = 5 plot lbepdf(x,2,5,c,d) for x = 1.01 0.01 2.99 . title Alpha = 5, Beta = 1 plot lbepdf(x,5,1,c,d) for x = 1.01 0.01 2.99 . end of multiplot . justification center move 50 97 text Log Beta Probability Density Functions Program 2: let alpha = 0.7 let beta = 2.1 let c = 1 let d = 10 let y = log beta rand numb for i = 1 1 500 let y2 x2 = binned y let amin = minimum y let amax = maximum y . title displacement 2 case asis title case asis label case asis . title Histogram with Overlaid PDF y1label Relative Frequency x1label X relative histogram y2 x2 limits freeze pre-erase off line color blue plot lbepdf(x,alpha,beta,c,d) for x = amin 0.1 amax limits pre-erase on line color black . title Log Beta Probability Plot y1label Theoretical x1label Data char x line bl log beta probability plot y justification center move 50 6 text PPCC = ^ppcc line solid char blank . multiplot corner coordinates 0 0 100 100 multiplot scale factor 2 y1label displacement 17 x1label displacement 12 multiplot 2 2 . let alpha1 = 0.5 let alpha2 = 5 let beta1 = 0.5 let beta2 = 5 set ppcc plot data points 100 . title PPCC Plot y1label Correlation Coefficient x1label Beta (Curves Represent Values of Alpha) log beta ppcc plot y let alpha = shape1 let beta = shape2 set ppcc plot axis order reverse log beta ppcc plot y set ppcc plot axis order default title Probability Plot y1label Theoretical x1label Data log beta probability plot y log beta kolmogorov smirnov goodness of fit y title label plot justification left move 25 90 text Alpha = ^alpha move 25 85 text Beta = ^beta move 25 80 text PPCC = ^ppcc move 25 75 text Min KS = ^statval end of multiplot . multiplot 2 2 let ksloc = 0 let ksscale = 1 title Chi-Square Plot y1label Minimum Chi-Square x1label Beta (Curves Represent Values of Alpha) log beta ks plot y2 x2 let alpha = shape1 let beta = shape2 set ppcc plot axis order reverse log beta ks plot y2 x2 set ppcc plot axis order default title Probability Plot y1label Theoretical x1label Data log beta probability plot y2 x2 log beta chi-square goodness of fit y2 x2 title label plot justification left move 25 90 text Alpha = ^alpha move 25 85 text Beta = ^beta move 25 80 text PPCC = ^ppcc move 25 75 text Min KS = ^statval end of multiplot
Date created: 8/23/2006 |