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What is Attribute Data and Variable Data?

Quality Glossary Definition: Attribute data

Also called: go/no-go information

Attribute data is defined as information used to create control charts. This data can be used to create many different chart systems, including percent charts, charts showcasing the number of affected units, count-per-unit charts, demerit charts, and quality score charts.

Quality Glossary Definition: Variable data

Also called: measurement information

Variable data is defined as information and figures used to build control charts. Variable data can be used to create average (X-bar) charts, range charts, and sample standard deviation charts or "S-charts."

How are Attribute & Variable Data Used to Create Control Charts?

When constructing a control chart to map out variances in measurement data, companies and teams must collect both attribute and variable data as a part of the problem-solving process.

Because the level of sensitivity of a measurement depends on the precision of the measuring device, there are times when variable data can be treated as attribute data.

For example, a company produces aluminum pins that may be smaller than 1.065 inches in diameter but not larger. Rather than measuring each pin or even a sample of pins, one can:

  • Use a plate that has a hole 1.065 inches in diameter bored through it (a go/no-go gauge)
  • Insert each pin to be inspected in the hole
  • Classify any pin that passes through the hole as accept, treating others as rejects

Thus, treating the variable as an attribute offers an efficient way to determine if the pin will be effective.

Establishing Performance Measures with Variable & Attribute Data

Performance measures, also known as process metrics or key quality indicators, should be ratios. These ratios are the statistics that describe how well or how poorly a process is performing.

Sometimes the ratios have labels such as defects per unit (dpu), defects per defective unit, defects per X units, defects per million opportunities (dpmo), or parts per million (ppm) defect rate. However, there are ratios that do not have labels. For example, the process capability index (CP) and the mean-sensitive process capability index (CPK) use unlabeled ratios.

Variables of Process Control: Attribute Data and Variable Data
Variables of Process Control: Attribute Data and Variable Data

Adapted from Essentials: A Reference Guide from A to Z, ASQ Quality Press.

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