Six Sigma Glossary from MiC Quality
Control Chart Types
 c Charts

These attribute charts are used to plot the number of nonconformities per unit (rather than units nonconforming) when the sample size is constant. The control limits are:

 Upper Control Limit Lower Control Limit

 CuSum Charts

Cusum Charts plot the cumulative sum of the deviations from a target value. They are useful for identifying process changes in process improvement activities:

V Masks can be used with Cusum Charts to measure the slope. The mask is placed on the last point. If points are outside the arms of the V, then a special cause is assumed.

 Demerits per Unit Chart

Used when the defects can be of varying severity. A weighting system is used to classify defects, more serious defects being given a higher weight.

The value plotted on the chart is calculated from the subgroup using:

Control limits are calculated from:

 Upper Control Limit Lower Control Limit

 EWMA Charts

Based on an exponentially weighted moving average. The control limits are:

 Steady State Upper Control Limit Lower Control Limit Initial Readings Upper Control Limit Lower Control Limit

L is the number of standard deviations

i is the number of periods

The 'Steady State' control limits are normally sufficient, however ten or so points are required before the smoothed average stabilizes on these values. The 'Initial Readings' limits should be used for the first few points plotted after a process change.

 Exponentially Weighted Moving Average Chart

See EWMA Charts

 Given Value Charts

These are sometimes used when the process is well-established and the mean and standard deviation are known.

 X-Bar Chart Upper Control Limit Lower Control Limit R Chart Upper Control Limit Lower Control Limit

Note that this should not be used to impose desired mean and standard deviation values.

 ImR (Individual & Moving Range) Charts

Individual & Moving Range Charts, also called XmR Charts. See XmR Charts

 Median Charts

Not generally recommended, but save calculation as long as the subgroup size is an odd number:

 Upper Control Limit Lower Control Limit :

Where the factors are:

 n 2 3 4 5 6 1.88 1.19 0.8 0.69 0.55

It is not usual to use range charts with median charts, because it defeats the purpose of simple calculation. All the data values from the subgroup may be plotted on the chart to indicate the spread:

 Moving Average Control Charts

Moving Average charts plot a running average of the last 'w' periods. The control limits are:

 Upper Control Limit Lower Control Limit

 np Charts

These are attribute type charts used to plot units nonconforming when samples of equal size are taken from the process. The control limits are:

 Upper Control Limit Lower Control Limit

 p Charts

These are attribute type charts used to plot units nonconforming when the samples are not of equal size. This is often when samples form a natural grouping - for example the number of treatments in a hospital in a week.

 Upper Control Limit Lower Control Limit

Note that the control limits are dynamic, they depend on the size of the sample.

 Tabular CuSum Charts

The problem with using Cusum charts for continuous monitoring is that slight deviations between the target and actual process average will cause a drift. Tabular cusum charts provide a 'slack' or tolerance band around the target. Excursions within this band will not be recorded. The band is often 1 standard deviation wide:

 u Charts

These attribute charts are used to plot the number of nonconformities per unit (rather than units nonconforming) when the sample size varies. The control limits are:

 Upper Control Limit Lower Control Limit

Note that the control limits are dynamic, they depend on the size of the sample.

 U Charts

See Demerits per Unit Charts

 X-Bar & R Charts

These variable charts, used as a pair, plot the mean and range of subgroups taken from the process. See 'Rational Subgroups' for guidance on how to select subgroups.

The control limits are calculated from:

 X-Bar Charts Upper Control Limit Lower Control Limit R Charts Upper Control Limit Lower Control Limit

The factors are taken from standard tables.

 X-Bar & S Charts

These variable charts, used as a pair are more efficient than X-Bar and R Charts, particularly for larger sample sizes:

 X-Bar Chart Upper Control Limit Lower Control Limit S Chart Upper Control Limit Lower Control Limit

 XmR Charts

Individual and Moving Range Charts. Used when the subgroup size is one. This may be when logistic reasons prevent larger subgroups, but is more commonly when there is no basis for rational subgroups.

The chart is based on the moving range between consecutive samples:

 X (Individual) Chart Upper Control Limit Lower Control Limit Moving Range Chart Upper Control Limit Lower Control Limit

D4 and d2 are taken from tables of standard factors, based on a subgroup size of 2:

 D4 3.267 d2 1.128

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