Quality Advisor

A free online reference for statistical process control, process capability analysis, measurement systems analysis,
control chart interpretation, and other quality metrics.

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Data Analysis Tools

Tools for analyzing and interpreting data so that areas to improve become apparent.

What type of data do I have?

Variables charts (measurement data)

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Consists of measurements of a characteristic, such as length, weight, density, time, or pressure.

Control charts Is your process stable and in control?
X-bar & range Use this if your data has a subgroup size of 2-10 observations.
X-bar & sigma Use this if your data has a subgroup size of 11 or more observations.
X-MR Use this if your data has a subgroup size of 1 observation.
Median Use this to analyze measurement data when you want to plot all observations.
Run chart Use this to see trends and patterns if there is not enough data for a control chart.
Histogram Use this to determine if your data has a normal distribution.
Capability analysis Use this to determine if your process is capable of producing output within specification limits.

 

Attributes (counts data)

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Consists of defects per item (nonconformities) or the number of defective items (nonconforming). For example, the number of non-working parts in sample or the number of blemishes counted on an individual part.

Control charts Is your process stable and in control?
np-chart Use this if your data is a count of nonconforming units and the subgroups are all the same size.
p-chart Use this if your data is a count of nonconforming units and the subgroup size varies.
c-chart Use this if your data is a count of nonconformities and the subgroups are all the same size.
u-chart Use this if your data is a count of nonconformities and the subgroup size varies.
Capability analysis Use this to determine capability for attributes data.

 

Pareto (counts in categories)

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Consists of a count of items or occurrences, such as the number of defective items, the number of scratches on a door panel, or how often a specific problem occurs.

Pareto diagram Use this to analyze counts that are in categories.

 

Rare event

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Use this when other control charts are not effective to determine if your process is stable.

g-chart Use this if your count data occurs infrequently. It is used by counting the number of events between rarely-occurring error or a nonconforming incident.
t-chart Use this if your error or non-nonconforming incident occurs infrequently. Each point on the chart represents an amount of time that has passed since the prior nonconforming incident occurred.

 

Interpreting quality charts

Control charts (both variables and attributes)

 

 

Step-by-step interpretation

Answer “yes” or “no” to a series of questions about your control charts.

Histogram

Follow these steps to interpret histograms.

  1. Study the shape.
  2. Calculate descriptive statistics.
  3. Compare the histogram to the normal distribution.

 

Other charts