SPC Glossary

Attribute Data

The qualitative data that can be counted for recording and analysis. Attribute data is usually gathered in the form of nonconforming units. The units are analyzed by P, nP, C and U control charts.

Attribute Quality Characteristic

A distinguishing feature of a process or its output on which attribute data can be collected.


The sum of values divided by the number, or sample size, of values. This is usually designated by a bar over the symbol for the values being averaged. The most common averages in SPC are:

 The average of values within a subgroup.

 The average of subgroup averages (also called the process average)

 The average of subgroup ranges.

Central Line

The line on the control chart that represents the average value of the items being plotted.


A distinguishing feature of a process on which variable data can be collected.

Control Chart

A graphic representation of a characteristic of a process. It shows the plotted values of data for the characteristic, a central line and one or two control limits.

Control Limit

A line, or lines, on a control chart used as a basis for judging the stability of a process. Control limits are calculated from process data.

Defect Data

Defect data consists of a defect code and a count of the number of units that contain that defect. It is possible for a unit to contain more than one defect. Defect data is displayed in a Pareto chart which ranks the defects by number of occurrences.

Defect Quality Characteristic

For CIMPLICITY SPC, a Defect Quality Characteristic defines the collection of defect categories be used for defect data for a Pareto chart. Other terms for Defect Quality Characteristic are Defect QC or defect data.


For CIMPLICITY SPC, a document is a collection of products for which you are going to collect and analyze variable quality characteristics and defect data.


See Average .


The middle value of a group of measurements when they are arranged in order from lowest to highest. The median is not an average.


An observation is a single instance of data.

Pareto Chart

A chart that ranks all potential problem areas according to their contribution to the total variation. Typically, a few causes account for most of the variation. In CIMPLICITY SPC, the contents of a Defects folder contain the data displayed in a Pareto chart.

Process Capability

The total range of a stable process's inherent variation. For variable data:

The process's inherent capability is defined as

The capability index for a stable process is

Process Spread

The extent to which the distribution of individual values of the process characteristic vary. This is often shown as the process average plus or minus some number of standard deviations. For example:


For CIMPLICITY SPC, a product is the entity for which you want to collect Variable quality characteristics or Defect data for analysis


The difference between the lowest and highest values in a subgroup.


A consecutive number of points consistently increasing or decreasing or above or below the central line.


The Greek letter - - used to designate a standard deviation.

Special Cause

A source of variation that is intermittent and often unpredictable. Sometimes called an assignable cause, it is signaled by a point beyond the control limits, a run, or other non-random pattern of points within the control limits.

Standard Deviation

A measure of the spread of the process output.


A subgroup is a set of one or more observations that are used to calculate a point on an SPC chart. Another term for a subgroup is a sample.

Variable Data

Variable data consists of measurable parameters such as weight, temperature, or length.

Variable Quality Characteristic

For CIMPLICITY SPC a Variable Quality Characteristic is a measurable parameter such as weight, temperature or length. Other terms for Variable Quality Characteristic are Variable QC or variable data.

More information

About Statistical Process Control.