Six Sigma Glossary

CAUSE-AND-EFFECT DIAGRAM: a tool for seeking out all the causes, regardless of magnitude, that are contributing to the effect, Y. It is also referred to as the Ishikawa diagram after developer Kaoru Ishikawa and the Fishbone Diagram due to its configuration as the skeleton of a fish. This diagram is one of the seven tools of quality and is one of frequently used subjective screening tools in a DMAIC Six Sigma project.

CENTRAL LIMIT THEOREM: States that given a distribution with a mean and variance, the sampling distribution of the mean appraches a normal distribution with a mean and variance/N as N, the sample size, increases. Also referred to as the Law of Large Numbers in the insurance and risk management field of study.

CHECK SHEET: "sheet" used to collect data in real-time and at the location where the data is generated. Data can be of any type and is the first steps to any improvement project which can be either quantitative or qualitative (attribute, discrete, variable, continuous).

COMMON CAUSES: causes of variation that are inherent in a process over time. They affect every outcome of the process and everyone working in the process. (See also "special causes.")

CONFORMANCE: an affirmative indication or judgment that a product or service has met the requirements of a relevant specification, contract, or regulation

CONTINUOUS IMPROVEMENT: the ongoing improvement of products, services, or processes through incremental and breakthrough improvements.

CONTROL CHART: a chart with upper and lower control limits on which values of some statistical measure for a series of samples or subgroups are plotted. The chart frequently shows a central line to help detect a trend of plotted values toward either control limit, the LCL or UCL.

CONTROL LIMIT: control limit for points above the central line in a control chart. There is a lower control limit (LCL) and an upper control limit (UCL) and this is provided by the process where as the specification limits (used in process capability assessments) are provided by the customer.

CORRECTIVE ACTION: the implementation of solutions resulting in the reduction or elimination of an identified problem.

CRITICAL TO QUALITY (CTQ): key measurable characteristics of a product or process whose performance standards, or specification limits, must be met in order to satisfy the customer. Sought in the DEFINE phase of Six Sigma projects as a part of gathering the Voice of the Customer (VOC). They align improvement or design efforts with critical issues that affect customer satisfaction.

CUMULATIVE SUM CONTROL CHART: a control chart on which the plotted value is the cumulative sum of deviations of successive samples from a target value. The ordinate of each plotted point represents the algebraic sum of the previous ordinate and the most recent deviations from the target.

CUSTOMER DELIGHTER: delighter features are used in Kano Model that are characteristics that pleasantly surprise and exceed customer expectations.

CUSTOMER SATISFACTION: the result of delivering a product or service that meets customer requirements.

DEFECT: a non-conforming characteristic for a product, process, or service. A defective unit may have one or more defects.

Causes that have higher lower class of defect are often given higher subjective scores relative to their impact on the effects. Such as "severity" scoring withing the FMEA creation process.

FACTOR: also known as a predictor variable (x, PIV). This input variable may be controlled or uncontrolled variable whose influence is being studied.

F-DISTRIBUTION: The F-value is a measurement of distance between individual distributions. As F goes up, P goes down saying that there is more confidence in there being a difference between two means and to accept the alternative hypothesis. To calculate take the Mean Square of X divided by the Mean Square of Error Compare this value to the F-critical value found in a table is another way to test hypothesis used in ANOVA.

FIT: Predicted value of the response variable provided a specific combination of factor settings.

GANTT CHART: Visual project planning device used for production scheduling. It is a horizontal bar chart that serves as a visual tool for project management. It illustrates dependent steps and where the project is at any given time. The chart was developed by Henry Laurence Gantt, born in 1861.

INFERENCE SPACE: Operating range of factors that are being analyzed.

KURTOSIS: measure of peakness or flatness of a distribution.

LINEARITY: Variation between a known standard across the low and high end of the gage. It is the difference between an individual's measurements and that of a known standard or truth over the full range of expected values.

MULTICOLLINEARITY: when two or more predictor variables (x, input variables) are found to be correlated with each other.

R-SQUARED: The amount of variation explained by the regression equation. Used in inference space to predict future outcomes on the basis of other related information. It is the sum of the squares of the regression model divided by total sum of squares. The square root of it is the correlation coefficient, r.

RESIDUAL: Difference between an actual and fitted (predicted) experimental value. Term commonly found when using regression or multiple regression.

RESPONSE VARIABLE (Y, KPOV): process output. This is linked to the customer Critical To Quality (CTQ) characteristics.

SEVEN TOOLS OF QUALITY: commonly used tools in just about every DMAIC Six Sigma project to help understand an existing process and drive most effective improvements.

SHEWHART CYCLE: named after father of statistical control and creator of the cotnrol chart, Walter Shewhart. Also called "plan-do-check-act cycle", or PDCA cycle, or PDSA cycle representing "plan-do-study-act".

SIGNAL-TO-NOISE (S/N) RATIO: a mathematical equation that indicates the magnitude of an experimental effect above the effect of experimental error due to chance fluctuations

SIX-SIGMA QUALITY: a term used to generally indicate that a process is well within specifications or has received "perfection". Process can be better than six-sigma. Technically the specification range is ±6 standard deviations.

SPC: Statistical Process Control

SPECIAL CAUSES: causes of variation that arise because of non-inherenent and special circumstances. Special causes are also referred to as assignable causes that will result initially a process being out of control. Not all special causes are found outside of the process control limits. Two types of variation: common cause and special cause

SPECIFICATION: a document or customer provided evidence that states the requirements to which a given product or service must conform. Commonly found as the lower specification limit (LSL) and/or the upper specification limits (USL). Customer may also provide a target that may or may not be in the middle of the LSL and USL.

STABILITY: represents variation due to elapsed time. It is the difference between an individual's measurements taken of the same parts after an extended period of time using the same techniques.

U CHART: count per unit chart used with attribute data

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