Transforming the Exponential for SPC Applications

Abstract

The long-tailed, positively skewed exponential distribution can be made into an almost symmetric distribution by taking the fourth root of the data. The transformed data can then be plotted conveniently on an individuals chart, an exponentially weighted moving average chart, or a cumulative sum chart for statistical process control (SPC). The log transformation of the exponential distribution, used for proper analysis of variance calculations, does stabilize the variance, but produces a very negatively skewed distribution that is not suitable for SPC applications. The observations for the fourth root can be easily calculated with a pocket calculator by depressing the square root key twice.

• Cumulative Sum Control Charts
• Exponential Distribution
• Exponentially Weighted Moving Average Control Charts
• Individuals Control Charts
• Poisson Distribution
• Shewhart Control Charts
• Statistical Process Control

Additional information

Notes on contributors

Rudolf G. Kittlitz

Rudy Kittlitz is a Senior Research Associate in DuPont Nylon Technical. He is a Fellow of ASQ. His email address is [email protected].