
DISTRIBUTIONAL BOOTSTRAPName:
One limitation of this method is that it does not provide a method for finding uncertainty intervals for these estimates. To address this, we have extended the BOOTSTRAP PLOT command to support a number of probability distributions. The bootstrap is a nonparametric method for calculating a sampling distribution for a statistic. The bootstrap calculates the statistic with N different subsamples. The subsampling is performed with replacement. To apply the bootstrap to the univariate distributional modeling problem, we do the following:
For the bootstrap plot, there will be a separate curve drawn for each parameter estimated. In addition, there will be a curve drawn for the PPCC value (or the KolmogorovSmirnov statistic). The vertical axis contains the computed value of these estimated parameters and the horizontal axis contains the sample number (for k = 1, 2, ..., N). The bootstrap plot is typically followed by some type of distributional plot, such as a histogram, for each estimated parameter. This is demostrated in the Program sample below. Dataplot also supports bootstrap computations for the case when there is one group variable. In this case, the horizontal axis is group id and the vertical axis contains the computed values of the estimated parameters for that group (the parameters are offset horizontally). The number of bootstrap samples is applied to each group. For example,if the requested number of bootstrap samples is 100, then each group will have 100 bootstrap samples applied.
where <y> is the response variable; <dist> is one of the following distributions:
ARCSINE CAUCHY COSINE EXPONENTIAL GUMBEL (EXTREME VALUE TYPE 1) HALF CAUCHY HALF LOGISTIC HALF NORMAL HYPERBOLIC SECANT LAPLACE (DOUBLE EXPONENTIAL) LOGISTIC NORMAL RAYLEIGH SEMICIRCULAR SLASH UNIFORM This syntax estimates the location and scale parameters for each bootstrap sample using a probability plot.
<SUBSET/EXCEPT/FOR qualification> where <y> is the response variable; <x> is the censoring variable; <dist> is one of the distributions given for Synatx 1; and where the <SUBSET/EXCEPT/FOR qualification> is optional. This syntax estimates the location and scale parameters for each bootstrap sample using a censored probability plot. The censoring variable should contain a value of 1 to indicate a failure time and a value of 0 to indicate a censoring time.
where <y> is the response variable; <dist> is one of the following distributions:
BRADFORD CHI CHISQUARE DOUBLE GAMMA DOUBLE WEIBULL ERROR (SUBBOTIN) FATIGUE LIFE FOLDED T FRECHET GAMMA GENERALIZED EXTREME VALUE GENERALIZED HALF LOGISTIC GENERALIZED LOGISTIC GENERALIZED PARETO GEOMETRIC EXTREME EXPONENTIAL INVERTED GAMMA INVERTED WEIBULL LOG DOUBLE EXPONENTIAL (LOG LAPLACE) LOG GAMMA LOG LOGISTIC LOGNORMAL PARETO PARETO SECOND KIND POWER POWER NORMAL RECIPROCAL SKEW LAPLACE (SKEW DOUBLE EXPONENTIAL) T TUKEYLAMBDA VON MISES WALD WRAPPED CAUCHY WEIBULL This syntax estimates the shape parameter using a PPCC plot and then estimates the location and scale parameters using a probability plot for each bootstrap sample .
<SUBSET/EXCEPT/FOR qualification> where <y> is the response variable; <x> is the censoring variable; <dist> is one of the distributions given for syntax 3; and where the <SUBSET/EXCEPT/FOR qualification> is optional. This syntax estimates the shape parameter using a censored PPCC plot and then estimates the location and scale parameters using a censored probability plot for each bootstrap sample. The censoring variable should contain a value of 1 to indicate a failure time and a value of 0 to indicate a censoring time.
where <y> is the response variable; <dist> is one of the distributions given for syntax 3; and where the <SUBSET/EXCEPT/FOR qualification> is optional. This syntax estimates the shape parameter using a KS plot and then estimates the location and scale parameters using a probability plot for each bootstrap sample.
where <y> is the response variable; <dist> is one of the following distributions:
F FOLDED NORMAL GANDH INVERSE GAUSSIAN GENERALIZED GAMMA This syntax estimates the two shape parameters using a PPCC plot and then estimates the location and scale parameters using a probability plot for each bootstrap sample.
<SUBSET/EXCEPT/FOR qualification> where <y> is the response variable; <x> is the censoring variable; <dist> is one of the distributions given in syntax 6; and where the <SUBSET/EXCEPT/FOR qualification> is optional. This syntax estimates the two shape parameters using a censored PPCC plot and then estimates the location and scale parameters using a censored probability plot for each bootstrap sample. The censoring variable should contain a value of 1 to indicate a failure time and a value of 0 to indicate a censoring time.
where <y> is the response variable; <dist> is one of the following distributions:
EXPONENTIAL FOLDED NORMAL GUMBEL (EXTREME VALUE TYPE 1) LAPLACE (DOUBLE EXPONENTIAL) LOGISTIC NORMAL RAYLEIGH UNIFORM This syntax estimates the location and scale parameters using maximum likelihood for each bootstrap sample.
<SUBSET/EXCEPT/FOR qualification> where <y> is the response variable; <x> is the censoring variable; <dist> is one of the following distributions:
EXPONENTIAL This syntax estimates the location and scale parameters using maximum likelihood for each bootstrap sample. The censoring variable should contain a value of 1 to indicate a failure time and a value of 0 to indicate a censoring time.
where <y> is the response variable; <dist> is one of the following distributions:
FATIGUE LIFE GAMMA GENERALIZED PARETO GEOMETRIC EXTREME EXPONENTIAL INVERSE GAUSSIAN LOGNORMAL PARETO WEIBULL This syntax estimates the shape and scale parameters (the Beta, Pareto, and inverse Gaussian estimate the two shape parameters but no scale parameter) using maximum likelihood for each bootstrap sample.
<SUBSET/EXCEPT/FOR qualification> where <y> is the response variable; <x> is the censoring variable; <dist> is one of the following distributions:
CENSORED LOGNORMAL CENSORED WEIBULL This syntax estimates the location and scale parameters using maximum likelihood for each bootstrap sample. The censoring variable should contain a value of 1 to indicate a failure time and a value of 0 to indicate a censoring time.
where <y> is the response variable; <dist> is one of the following distributions:
This syntax estimates the shape, location, and scale parameters using maximum likelihood for each bootstrap sample.
BOOTSTRAP NORMAL MLE PLOT Y BOOTSTRAP WEIBULL PLOT Y BOOTSTRAP WEIBULL KS PLOT Y BOOTSTRAP WEIBULL CENSORED PLOT Y X
dpst1f.dat The estimates for each bootstrap sample are written to file dpst1f.dat. You can read the variables written to dpst1f.dat to generate histograms and to compute selected percentiles. The order is:
location parameter scale parameter first shape parameter second shape parameter ppcc (or ks) value If a particular syntax does not generate one or more of these values, then they are omitted (e.g., the normal distribution does not generate estimates for any shape parameters). The following example generates a Weibull bootstrap plot and then reads the bootstrap estimates from dpst1f.dat.
SKIP 0 READ DPST1F.DAT ALOC ASCALE AGAMMA APPCC MULTIPLOT 2 2 RELATIVE HISTOGRAM ALOC RELATIVE HISTOGRAM ASCALE RELATIVE HISTOGRAM AGAMMA RELATIVE HISTOGRAM APPCC END OF MULTIPLOT dpst2f.dat Selected percentiles are written to dpst2f.dat. The order is:
Parameter number  parameters are ordered as they are in dpst1f.dat Mean Standard Deviation Median 2.5 percentile 97.5 percentile 5.0 percentile 95.0 percentile 0.5 percentile 99.5 percentile
If you enter the command
you will get bootstrap estimates for the following percentiles:
If you would like to specify the specific percentiles to estimate, enter the command
with YPERC denoting a variable that contains the desired percentiles. This is demonstrated in the sample program below. By default, two sided confidence intervals are generated for the percentiles. The following commands can be used to generate either lower one sided or upper one sided intervals
SET BOOTSTRAP DISTRIBUTIONAL PERCENTILES UPPER To turn off the computation of the percentile confidence intervals, enter
To reset the default of two sided intervals, enter
LET GAMMA2 = 10 BOOTSTRAP WEIBULL PLOT Y One recommendation is to generate the bootstrap plot for a relatively small number of samples (e.g., 50) and use that to determine a reasonable range for the shape parameter. Enter HELP PPCC PLOT to see the relevant parameter for the desired distribution.
The BCA option is not currently supported for the bootstrap distributional plots.
The full sample is used to generate the parameter estimates of the distribution. Then the bootstrap samples are generated by generating random numbers from the specified distribution using the parameters estimated from the full sample. The following command will specify this alternate method of bootstrapping be used:
To restore the default method of bootstrapping the data values, enter the command
The parameteric option is still being tested and may not work correctly for some of the distributions.
MAXIMUM LIKELIHOOD is a synonym for MLE
Efron and Tibshirabi (1993), "An Introduction to the Bootstrap", SpringerVerlang.
. Following Sample Macro demonstrates the use of the . bootstrap with a Weibull distribution. . . Step 0: Create some sample Weibull data . dimension 50 columns . let gamma = 2.3 let y = weibull random numbers for i = 1 1 100 . . Step 1: Perform PPCC/Probability Plot Analysis, . Perform KS goodness of fit test . set ipl1na distboo1.ps device 2 postscript device 2 color on . multiplot 2 2 multiplot corner coordinates 0 0 100 100 multiplot scale factor 1.5 y1label displacement 12 . title displacement 2 x1label displacement 12 title Weibull PPCC Plot x1label Shape Parameter (gamma) y1label Correlation weibull ppcc plot y justification left height 3.5 move 25 28 text Max PPCC: ^maxppcc move 25 21 text Shape: ^shape let gamma1 = shape  2 if gamma1 <= 0 let gamma1 = 0.1 end of if let gamma2 = shape + 2 title Weibull PPCC Plot weibull ppcc plot y move 25 28 text Max PPCC: ^maxppcc move 25 23 text Shape: ^shape let gamma = shape if n <= 200 character x line blank else line solid character blank end of if . title Weibull Probability Plot x1label displacement x1label Theoretical y1label Data weibull probability plot y justification center move 50 2 text Location: ^ppa0, Scale: ^ppa1 let iplot = 3 multiplot 2 2 iplot line solid character blank limits freeze preerase off let function f = ppa0 + ppa1*x let zmin = minimum xplot let zmax = maximum xplot let ainc2 = (zmax  zmin)/10 plot f for x = zmin ainc2 zmax limits preerase on let iplot = iplot + 1 multiplot 2 2 iplot title Histogram with Overlaid Weibull x1label Data Units y1label Density relative histogram y multiplot 2 2 iplot limits freeze preerase off let loc2=ppa0 let scale2=ppa1 line solid char blank line color blue let amin = minimum y let amax = maximum y let ainc = 0.1 plot weipdf(x,shape,loc2,scale2) for x = amin ainc amax limits preerase on line color black delete gamma gamma1 gamma2 end of multiplot device 2 close . . Step 2: Now perform a bootstrap analysis . feedback off capture distboot.out write " " write "BOOTSTRAPBASED INTERVALS" write " " set maximum likelihood percentiles default bootstrap samples 200 let gamma1 = 0.2 let gamma2 = 5 set ipl1na distboo2.ps device 2 postscript device 2 color on multiplot 2 3 multiplot corner coordinates 0 0 100 100 multiplot scale factor 1.7 y1label Parameter Estimate x1label x2label Bootstrap Sample title Bootstrap Plot line color blue red green limits bootstrap weibull plot y line color black all . delete aloc ascale ashape appcc skip 0 read dpst1f.dat aloc ascale ashape appcc y1label x2label x3label displacement 16 title Location Parameter let amed = median aloc let amean = mean aloc let asd = sd aloc x2label Med = ^amed, Mean = ^amean x3label SD = ^asd histogram aloc title Scale Parameter let amed = median ascale let amean = mean ascale let asd = sd ascale x2label Med = ^amed, Mean = ^amean x3label SD = ^asd histogram ascale title Shape Parameter let amed = median ashape let amean = mean ashape let asd = sd ashape x2label Med = ^amed, Mean = ^amean x3label SD = ^asd histogram ashape title PPCC Value let amed = median appcc let amean = mean appcc let asd = sd appcc x2label Med = ^amed, Mean = ^amean x3label SD = ^asd histogram appcc x3label displacement . device 2 close . let alpha = 0.05 let xqlow = alpha/2 let xqupp = 1  alpha/2 . write "Bootstrapbased Confidence Intervals" write "alpha = ^alpha" write " " . let xq = xqlow let loc95low = xq quantile aloc let xq = xqupp let loc95upp = xq quantile aloc let xq = xqlow let sca95low = xq quantile ascale let xq = xqupp let sca95upp = xq quantile ascale let xq = xqlow let sha95low = xq quantile ashape let xq = xqupp let sha95upp = xq quantile ashape write "Confidence Interval for Location: (^loc95low,^loc95upp)" write "Confidence Interval for Scale: (^sca95low,^sca95upp)" write "Confidence Interval for Gamma: (^sha95low,^sha95upp)" . . Now generate confidence intervals for percentiles . serial read p 0.5 1 2.5 5 10 20 30 40 50 60 70 80 90 95 97.5 99 99.5 end of data let nperc = size p skip 1 read matrix dpst4f.dat xqp write " " loop for k = 1 1 nperc let xqptemp = p(k) let amed = median xqp^k let xqpmed(k) = amed let xq = xqlow let atemp = xq quantile xqp^k let xq95low(k) = atemp let xq = xqupp let atemp = xq quantile xqp^k let xq95upp(k) = atemp end of loop set table title "Bootstrap Based Confidence Intervals for Percentiles" set table spacing 15 set write decimals 7 write " " write "Confidence Intervals for Percentiles" write p xqpmed xq95low xq95upp end of capture delete xqp xqpmed xq95low xq95upp delete gamma1 gamma2 BOOTSTRAPBASED INTERVALS Bootstrapbased Confidence Intervals alpha = 0.05 Confidence Interval for Location: (0.4112,0.272742) Confidence Interval for Scale: (0.744203,1.572763) Confidence Interval for Gamma: (1.738776,3.748062) Confidence Intervals for Percentiles VARIABLESP XQPMED XQ95LOW XQ95UPP 0.5000000 0.1427522 0.0264724 0.3189068 1.0000000 0.1783569 0.0333498 0.3433715 2.5000000 0.2441821 0.1315464 0.3925191 5.0000000 0.3210546 0.2229533 0.4490436 10.0000000 0.4301926 0.3367559 0.5266151 20.0000000 0.5761956 0.4959525 0.6575613 30.0000000 0.6924722 0.6142470 0.7801363 40.0000000 0.8017390 0.7214258 0.8862920 50.0000000 0.9073263 0.8198442 0.9952904 60.0000000 1.0129241 0.9191304 1.1054595 70.0000000 1.1306920 1.0393872 1.2280047 80.0000000 1.2808006 1.1713773 1.3832619 90.0000000 1.4895294 1.3569634 1.5912455 95.0000000 1.6622919 1.5139780 1.7756084 97.5000000 1.8108935 1.6356139 1.9443940 99.0000000 1.9902619 1.7803770 2.1564791 99.5000000 2.1085887 1.8710965 2.3045762  
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Date created: 4/20/2005 