Template:Using contour plots

Using Contour Plots
To determine whether two data sets are significantly different and at what confidence level, one can utilize the contour plots provided in Weibull++. By overlaying two contour plots from two different data sets (analyzed using the same distribution) at the same confidence level, one can visually assess whether the data sets are significantly different at that confidence level if there is no overlap on the contours. The disadvantage of this method is that the same distribution must be fitted to both data sets.

Example 15
The following data represent the times-to-failure for a product. Certain modifications were made to this product in order to improve its reliability. Reliability engineers are trying to determine whether the improvements were significant in improving the reliability. $$\overset{\text{Old Design}}{\mathop{\begin{array}{*{35}{l}} \text{2} & \text{2} & \text{3} & \text{4} & \text{6} & \text{9} \\ \text{9} & \text{11} & \text{17} & \text{17} & \text{19} & \text{21} \\ \text{23} & \text{28} & \text{33} & \text{34} & \text{34} & \text{37} \\ \text{38} & \text{40} & \text{45} & \text{55} & \text{56} & \text{57} \\ \text{67} & \text{76} & \text{90} & \text{115} & \text{126} & \text{197} \\ \end{array}}}\,$$

$$\overset{\text{New Design}}{\mathop{\begin{array}{*{35}{l}} \text{15} & \text{32} & \text{61} & \text{67} & \text{75} \\ \text{116} & \text{148} & \text{178} & \text{181} & \text{183} \\ \end{array}}}\,$$

At what significant level can the engineers claim that the two designs are different?

Solution to Example 15
For both data sets the two-parameter Weibull distribution best fits the data. The contour plots were generated and overlaid on a MultiPlot.

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From this plot, it can be seen that there is an overlap at the 95% confidence level and that there is no overlap at the 90% confidence level. It can then be concluded that the new design is better at the 90% confidence level. If a analyst wanted to know at exactly what confidence the two contour plots meet, she would have to incrementally raise the confidence level from 90% until the two plots met. In fact, this search process has been automated by the Confidence Level Detector utility.

Confidence Level Detector
The Confidence Level Detector (CLD) utility is located as an option in the Contour Plot Setup window. To activate the utility, check the “Plot Critical Level” box as shown in the figure below.



The CLD utility determines the confidence level at which the contour plots of two data sets meet at a single point. This is the minimum confidence level at which the contour plots of the two different data sets overlap. At any confidence level below this minimum confidence level, the contour plots of the two data sets will not overlap and there will be a statistically significant difference between the two populations at that level.

Let and be the 99% two-sided lower bounds on for populations 1 and 2, respectively, and let and be the 99% two-sided upper bounds on for populations 1 and 2, respectively. 99% is used as the starting confidence level to make sure that the contour plots are large enough to overlap. The confidence levels are decreased, which in turn decreases the size of the contour plots of the two populations, causing the intersection points of the two contour plots move towards each other, as illustrated in the figure below.



The confidence level is decreased until the contour plots meet at a single point.