SPC Overview 211

attribute data

Data that represents a characteristic or individual count. Attribute data, also known as discrete data, cannot be added to or subtracted from other attribute data sets.

bell curve

A graph of continuous data characterized by a high center, tapered sides, and flared edges. A bell curve reflects conditions that exhibit natural variation.

centerline

The horizontal line on a control chart that represents the expected average for a process.

common cause variation

A deviation that is normal and expected. Common cause variation, or natural variation, cannot be traced back to a single source.

continuous data

Data that can be measured on a scale and compared with other data. Continuous data, also known as variable data, can be added to or subtracted from other continuous data.

control charts

A graph used in SPC to show trends in data over time. Control charts include upper and lower limits between which the process should perform.

data

Factual information that is used for analysis and problem solving. Data is often in the form of values or numbers.

discrete data

Data that represents a characteristic or individual count. Discrete data, also known as attribute data, cannot be added to or subtracted from other discrete data sets.

external customers

An outside organization or individual that receives a product or service from a company. External customers dictate a product's key quality characteristics.

frequency distribution chart

A chart that displays the number of occurrences in a continuous data set. On a frequency distribution chart, common cause variations typically create a bell curve.

grand average

The expected average of all sample averages. The grand average is the centerline on an X bar chart.

internal customers

A department or individual within a company that relies on others to satisfy the external customer. For any cell, the next cell in a process is always the internal customer.

key quality characteristics

A measurable characteristic of a product that greatly impacts customer satisfaction. Key quality characteristics are the focus of SPC efforts.

lower control limit

LCL. A control limit on a control chart indicating the boundary for the minimum permissible values. On an X bar chart, the lower control limit is located at -3 sigma.

mean

The average of a numerical set. A mean is calculated by dividing the sum of a set of numbers by the number of members in the group.

natural variation

A deviation that is normal and expected. Natural variation, also known as common cause variation, cannot be traced back to a single source.

normal distribution

Variable data that clusters about an average and is symmetrical. When graphed, a normal distribution appears as a bell-shaped curve. In-control processes yield a normal distribution.

P chart

The control chart that tracks the percentage of nonconforming items. P charts are used to track attribute data.

P chart

The control chart that tracks the percentage of nonconforming items. P charts are used to track discrete data.

process capability

The ability of a process to consistently meet a set of specific limitations. Process capability involves the total range of variation within a process, including common and special causes.

process control

A method of monitoring process performance by measuring and regulating the processes that yield a product. Process control involves identifying and reducing sources of special cause variation.

processes

A series of activities required to complete a product or provide a service to a customer. Manufacturers monitor process to ensure product quality.

quality

The satisfaction of customer requirements. Quality products conform to specifications, are free of defects, and meet the requirements of their anticipated use.

R chart

The control chart that tracks sample ranges over time. R charts are usually used with X bar charts to track variable data.

range

The difference between the smallest and largest values in a numerical set. In SPC, R charts are used to track sample ranges.

ranges

The difference between the smallest and largest values in a numerical set. In SPC, R charts are used to track sample ranges.

sigma

A unit of standard deviation indicating the degree of spread within a set of measurements. On an X bar chart, the upper control limit is located at +3 sigma, and the lower control limit is located at -3 sigma.

SPC

Statistical process control. The use of statistics and control charts to measure key quality characteristics and control how the related process behaves. SPC separates special causes of variation from common causes.

special cause variation

A deviation that causes an undesirable, fundamental change in a process. Special cause variation, also known as unnatural variation, can be traced back to a single source.

statistical process control

SPC. The use of statistics and control charts to measure key quality characteristics and control how the related process behaves. SPC separates special causes of variation from common causes.

statistics

The science of collecting, summarizing, and analyzing numerical data. Statistics makes it possible to predict the likelihood of events.

tolerance

A specification indicating an unwanted but acceptable deviation from a given dimension. Part tolerances are indicated on blueprints.

unnatural variation

A deviation that causes an undesirable, fundamental change in a process. Unnatural variation, also known as special cause variation, can be traced back to a single source.

upper control limit

UCL. A control limit on a control chart indicating the boundary for the maximum permissible values. On an X bar chart, the upper control limit is located at +3 sigma.

variable data

Data that can be measured on a scale and compared with other data. Variable data, also known as continuous data, can be added to or subtracted from other variable data sets.

variation

Any deviation from what is normal and consistent. In manufacturing, variation from what is normal can signal that an error has occurred.

X bar chart

A chart used to track a series of sample averages. X bar charts are usually used with R charts to track variable data.