Bias of Estimator for Cpk
/****************************************************************/
/* S A S S A M P L E L I B R A R Y */
/* */
/* NAME: CPKBIAS */
/* TITLE: Bias of Estimator for Cpk */
/* PRODUCT: QC */
/* SYSTEM: ALL */
/* KEYS: Capability Analysis, Capability Indices, */
/* PROCS: G3D */
/* DATA: */
/* */
/* REF: W. L. Pearn, S. Kotz, N. L. Johnson (1992). */
/* "Distributional and Inferential Properties of */
/* Process Capability Indices". Journal of Quality */
/* Technology 24, pp. 216-231. */
/* */
/* Rodriguez, R. N. (1992). (Recent Developments in */
/* Process Capability Analysis". Journal of Quality */
/* Technology 24, pp. 176-187. */
/* */
/* NOTES: This program calculates the bias for the estimator */
/* of Cpk using results of Pearn et al. (1992). Also */
/* see page 178 of Rodriguez (1992). The bias is */
/* compuated as a function of standardized parameters */
/* d = ( USL - LSL ) / 2 */
/* | Mu - M | */
/* where M=(USL+LSL)/2. */
/* */
/* MISC: */
/* */
/****************************************************************/
options ps=60 ls=80;
data cpk;
label bi = 'Bias'
ms = '|Mu-M|/Sigma'
ds = 'd/Sigma';
keep n ms ds ex bi;
n = 30;
f = n-1;
f1 = (1/3)*sqrt(f/2)*gamma((f-1)/2)*(1/gamma(f/2));
do ms = 0.0 to 2.0 by 0.05;
f2 = sqrt(2/n)*(1/gamma(0.5))*exp(-n*0.5*ms*ms);
f3 = ms*(1-(2*probnorm(-sqrt(n)*ms)));
do ds = 2 to 6 by 0.05;
ex = f1*(ds-f2-f3);
bi = ex - ((ds-ms)/3);
output;
end;
end;
run;
title 'Bias of Cpk Estimator for n = 30';
proc g3d data = cpk;
plot ds*ms = bi /
xticknum = 5
yticknum = 5
zticknum = 5
zmin = -0.05
zmax = 0.05
rotate = 45
tilt = 70
grid
;
run;
goptions reset=all;