Meeker and Escobar (1998, p. 447) analyzed data from an accelerated test on the lifetimes of glass capacitors as a function of operating voltage and temperature. The following SAS statements create a SAS data set containing the data. There are four lifetimes for each of eight combinations and four censored observations after the fourth failure for each combination:
data glass; input Temp Voltage @; do i = 1 to 4; cen = 0; input Hours @; output; end; do i = 1 to 4; cen = 1; output; end; datalines; 170 200 439 904 1092 1105 170 250 572 690 904 1090 170 300 315 315 439 628 170 350 258 258 347 588 180 200 959 1065 1065 1087 180 250 216 315 455 473 180 300 241 315 332 380 180 350 241 241 435 455 ;
The following statements analyze the capacitor data. The MODEL statement fits a regression model with Temp
and Voltage
as independent variables. Parameter estimates from the fitted regression model are shown in Figure 16.24. An interaction term between Temp
and Voltage
is included. The PPLOT statement creates a Weibull probability plot shown in Figure 16.25 with all temperature-voltage combinations overlaid on the same plot. The regression model fit is also plotted. The RPLOT
statement creates the plot shown in Figure 16.26 of the data and Weibull distribution percentiles from the regression model as a function of voltage for values of temperature
of 150, 170, and 180:
proc reliability data = glass; distribution Weibull; model Hours*cen(1) = temp voltage temp * voltage; pplot Hours*cen(1) = ( temp voltage ) / fit = model overlay noconf lupper = 2000; run;
proc reliability data = glass; distribution Weibull; model Hours*cen(1) = temp voltage temp * voltage; rplot Hours*cen(1) = voltage / fit = regression(temp = 150, 170, 180) plotfit; run;
Figure 16.24: Parameter Estimates for Fitted Regression Model
Weibull Parameter Estimates | ||||
---|---|---|---|---|
Parameter | Estimate | Standard Error | Asymptotic Normal | |
95% Confidence Limits | ||||
Lower | Upper | |||
Intercept | 9.4135 | 10.5402 | -11.2449 | 30.0719 |
Temp | -0.0062 | 0.0598 | -0.1235 | 0.1110 |
Voltage | 0.0086 | 0.0374 | -0.0648 | 0.0820 |
Temp*Voltage | -0.0001 | 0.0002 | -0.0005 | 0.0003 |
EV Scale | 0.3624 | 0.0553 | 0.2687 | 0.4887 |
Weibull Shape | 2.7593 | 0.4210 | 2.0461 | 3.7209 |
Figure 16.25: Probability Plot for Glass Capacitor Regression Model
Figure 16.26: Plot of Data and Fitted Weibull Percentiles for Glass Capacitor Regression Model