This is just an Excerpt from a larger document, click here to view the entire document.ESS Monitoring
Statistical Process Control (SPC) and Pareto charts are the primary tools for monitoring ESS performance. Prepared from data contained in the FRACAS database, these reports are used to monitor key ESS parameters against established requirements. Typical key ESS parameters include Incoming Latent Defects, Stress Screening Strength, Latent Defects Remaining, Defect Trend Analyses, and Field Failures Observed.
The SPC chart presents a graphical comparison of actual results to requirements. The expected statistical variation due to sample size is calculated using a Poisson distribution. This variation, commonly expressed as ±3 standard deviations, is plotted along with the actual results.
Pareto charts are used to display breakdowns of failure causes and are useful for showing defect frequency. Often, the actual results reported on a Pareto are guard banded by the high and low expected results.
The ESS process is a closed-loop process and relies upon information obtained through monitoring to valuate and improve the implemented screens. It is only through this feedback that the ESS program can remain balanced in terms of effective latent defect removal and cost. As stated earlier ESS is not a test and should not be viewed as having rigid requirements. With time, new key parameters may surface and previous key parameters may no longer be of concern. Changes in manufacturing techniques may eliminate some latent defects and introduce new ones. To remain effective, the ESS program must evolve.