Six Sigma Statistical Terms - IX

Six Sigma Statistical Terms
SAMPLE
A portion of a population of data chosen to estimate some characteristic about the whole population. One or more observations drawn from a larger collection of observations or universe (population).

SCATTER DIAGRAMS
Charts which allow the study of correlation, e.g., the relationship between two variables or data sets.

SHORT RUN STATISTICAL
A statistical control charting technique which applies to

PROCESS CONTROL
Any process situation where there is insufficient frequency of subgroup data to use traditional control charts (typically associated with low-volume manufacturing or where setups occur frequently). Multiple part numbers and multiple process streams can be plotted on a single chart.

SIX M’S
The major categories that contribute to effects on the fishbone diagram (man, machine, material, method, measurement, and mother nature.

SIX SIGMA
A term coined by Motorola to express process capability in parts per million. A Six Sigma process generates a maximum defect probability of 3.4 parts per million (PPM) when the amount of process shifts and drifts are controlled over the long term to less than +1.5 standard deviations.

SKEWED DISTRIBUTION
A non-symmetrical distribution having a tail in either a positive or negative direction.

SPECIAL CAUSE See ASSIGNABLE CAUSE.

STABLE PROCESS
A process which is free of assignable causes, e.g., in statistical control.

STANDARD DEVIATION
A statistical index of variability which describes the process spread or width of distribution.

STATISTICAL CONTROL
A quantitative condition which describes a process that is free of assignable/special causes of variation (both mean and standard deviation). Such a condition is most often evidenced on a control chart, i.e., a control chart which displays an absence of nonrandom variation.

STATISTICAL PROCESS
The application of standardized statistical methods and CONTROL (SPC) procedures to a process for control purposes.

SUBGROUP
A logical grouping of objects or events which displays only random event-to-event variations, e.g., the objects or events are grouped to create homogenous groups free of assignable or special causes. By virtue of minimizing within subgroup variability, any change in the central tendency or variance of the universe will be reflected in the “subgroup-to-subgroup” variability. A predetermined sample of consecutive parts or other data bearing objects removed from the process for the purpose of data collection. SYMPTOM That which serves as evidence of something not fully understood in factual terms.

SYSTEM
That which is connected according to a scheme.

SYSTEMATIC VARIABLES
A pattern which displays predictable tendencies.

TEST OF SIGNIFICANCE
A statistical procedure used to determine whether or not a process observation (data set) differs from a postulated value by an amount greater than that due to random variation alone.

THEORY
A plausible or scientifically acceptable general principle offered to explain phenomena.

TWO-SIDED ALTERNATIVE
The values of a parameter which designate both an upper and lower bound.

TYPE I ERROR See ALPHA RISK.

TYPE II ERROR See BETA RISK.