SPC Glossary: C

Quality Management System Glossary: C


The comparison of a measurement instrument or system of unverified accuracy to a measurement instrument or system of known accuracy to detect any variation from the required performance specification.


The total range of inherent variation in a stable process; determined by using data from control charts.


An identified reason for the presence of a defect or problem.

Cause-and-Effect Diagram

Also called a fishbone diagram or an Ishikawa diagram (after its developer).

A quality control tool used to analyze potential causes of problems in a product or process.


See Count Chart.


A line on a graph that represents the overall average (mean) operating level of the process.

Central Limit Theorem

Also known as Central Limit Theorem Formula.

An important statistical theorem that states that subgroup averages tend to be normally distributed even if the output overall is not. This concept allows control charts to be widely used for process control even if the underlying process is not normally distributed.

Central Tendency

Also known as Measures of Central Tendency.

The tendency of data gathered from a process to cluster toward a middle value somewhere between the high and low values of measurement.


A factor, element, or measure that defines and differentiates a process, function, product, service, or other entity.


A tool for organizing, summarizing, and depicting data in graphic form.

Check Sheet

A simple data recording device. The check sheet is custom-designed by the user, which allows him or her to readily interpret the results.

Classification of Defects

The listing of possible defects of a unit, classified according to their level of severity. Commonly used classifications include: A, B, C, or D; critical, major, minor, or incidental; and critical, major, or minor. A separate acceptance sampling plan is generally applied to each class of defects.

Common Cause

Cause of variation that is inherent in a process over time. A common cause affects every outcome of the process and everyone working in the process. Also see Special Cause.

Consumer’s Risk

Pertains to sampling and the potential risk that bad products will be accepted and shipped to the consumer.

Continuous Flow Process

A method of manufacturing that aims to move a single unit in each step of a process, rather than treating units as batches for each step.

Continuous Improvement (CI)

Also known as Continuous Quality Improvement and Continual Improvement.

The ongoing improvement of products, services, or processes through incremental (over time) and/or breakthrough (all at once) improvements.

Continuous Sampling

Used when the product is manufactured in a continuous flow and is not able to be grouped into lots (batches). Two parameters are considered: Frequency (f) and Clearing Number (i). This is a progressive type of plan in which the Clearing Number is X (example = 60) and the frequency is 1/X (example = 1/20). The manufacturer inspects 100 percent of the product until (i)=60 is reached. If defect-free, the Frequency (example = 1/20) applies and now every (f)=20th sample is inspected. If at least one defect is found in the first (i)=60, 100-percent inspection continues until the Clearing Number is reached.

Control Chart

A graph used to study how a process changes over time. Frequently shows a central line to help detect a trend of plotted values toward either upper or lower Control Limit.

Control Limit

Also known as Process Control Limit and Natural Process Limit.

The boundaries of a process within specified confidence levels expressed as the Upper Control Limit (UCL) and the Lower Control Limit (LCL).

Control Plan (CP)

Written descriptions of the systems for controlling part and process quality by addressing the key characteristics and engineering requirements.

Corrective Action

A solution meant to reduce or eliminate an identified problem.

Correlation (Statistical)

A measure of the relationship between two data sets of variables.

Costs of Poor Quality (COPQ)

The costs that would disappear if systems, processes, and products were perfect. These costs are organized into four categories: internal failure costs (costs associated with defects found before the customer receives the product or service); external failure costs (costs associated with defects found after the customer receives the product or service); appraisal costs (costs incurred to determine the degree of conformance to quality requirements); and prevention costs (costs incurred to keep failure and appraisal costs to a minimum).

Cost of Quality (COQ)

A means to quantify the total cost of quality-related efforts and deficiencies. Considered by some to be synonymous with COPQ.

Count Chart

A Control Chart for evaluating the stability of a process in terms of the count of events of a given classification occurring in a sample. Commonly referred to as a c-chart.

Count Data

See Attribute Data.

Count-Per-Unit Chart

Also known as a u-chart.

A type of control chart used to monitor count-type data where the sample size is greater than one, typically the average number of nonconformities per unit.


A measure of dispersion, sometimes described as the engineering tolerance divided by the natural tolerance. The ratio of tolerance to 6 sigma (i.e., the Upper Specification Limit [USL], minus the Lower Specification Limit [LSL], divided by 6 sigma.

Cpk Index

Also known as Process Capability Index.

Equals the lesser of the Upper Specification Limit minus the mean divided by 3 sigma or the mean minus the Lower Specification Limit divided by 3 sigma. The greater the Cpk value, the better.

Cumulative Sum Control Chart (CUSUM)

A type of control chart used to monitor small shifts in the process mean. It uses the cumulative sum of deviations from a target.