Statistical Process Control
The Nevada chart format includes a statistical process control (SPC) method for analyzing warranty returns data. The SPC method evaluates the discrepancy between the actual number of failures and the number of failures predicted by the distribution parameters. It uses a chi-squared test to help you detect unusually high or low return rates for any given period, alerting you to any possible deviations in manufacturing, quality control or any other factors that may adversely affect the reliability of the product in the field. This enables you to intervene immediately and avoid increased warranty costs or more serious repercussions. The SPC application is available only for the Nevada chart format.
Note: The SPC feature applies only to homogeneous data sets and is therefore unavailable when you select the Use Subsets option on the Main page of the control panel.
To use SPC, click the Statistical Process Control page icon on the control panel.
Select the Calculate chi-squared values check box, and then enter the desired control values into the Critical and Caution fields in decimal form.
The control values are the upper probability levels of the chi-squared distribution below which the sales or return periods will be flagged as outliers. The values must be greater than zero and less than one. By default, the critical value is 1% and the caution value is 10%.
Select the Color-code Returns sheet check box to highlight the results. You can select to evaluate the results By sales period (results are color-coded by rows) or By returns period (results are color-coded by columns).
Green indicates that the number of product returns is within the expected range.
Yellow indicates that the number of product returns is between the caution and critical levels.
Red indicates that the number of product returns is outside the expected range.
If you have manually changed the colors of any cells, click the Restore Guide Colors button to return the cells to their original colors.
Tip: If you wish to perform a manual evaluation of the table of chi-squared values, you can use Weibull++’s Quick Statistical Reference (QSR) tool to look up chi-squared values for different control values and degrees of freedom. You can access the QSR by choosing Home > ReliaSoft > Quick Tools > Quick Statistical Reference.