---
alias: user-guide-chart-types-index
description: "This documentation outlines eight basic SPC chart types for process monitoring and control"
---

# Chart Types

A SPC **Chart** in Critical Manufacturing can be of eight basic types:

* Variable Charts:
    * Average and Range (x̄ R)
    * Average and Standard Deviation (x̄ s)
    * Median and Range (x~ R)
    * Individuals and Moving Range (x Rm)

* Attribute Charts:
    * Fraction Defective (p)
    * Number Defective (np)
    * Number of Defects Per Unit (u)
    * Number of Defects (c)

The variable charts assume continuos values that follow a normal distribution, whereas attribute charts have discrete values and follow other distributions:

* p and np charts follow a Binomial distribution
* u and c charts follow a Poisson distribution

The following table describes the properties of each chart type

| Chart | Process observation types | Process observation relationships | Sample Size | Distribution type | Size of shift to detect |
|---|---|---|---|---|---|
| Average and Range (x&#772; R) | Variables | Independent | < 10, but usually 3 to 5 | Normal | Large (&ge;1.5&#963;) |
| Average and Standard Deviation (x&#772; s) | Variables | Independent | Usually &ge; 10 | Normal | Large (&ge;1.5&#963;) |
| Median and Range (x&#126; R) | Variables | Independent | < 10, but usually 3 to 5 | Normal | Large (&ge;1.5&#963;) |
| Individuals and Moving Range (I MR) | Variables | Independent | 1 | Normal | Large (&ge;1.5&#963;) |
| Fraction Defective (p) | Attributes | Independent | Variable usually &ge; 50 | Binomial | Large (&ge;1.5&#963;) |
| Number Defective (np) | Attributes | Independent | Constant usually &ge; 50 | Binomial | Large (&ge;1.5&#963;) |
| Number of Defects per Unit (u) | Attributes | Independent | Variable | Poisson | Large (&ge;1.5&#963;) |
| Number of Defects (c) | Attributes | Independent | Constant | Poisson | Large (&ge;1.5&#963;) |

Table: Chart type properties

## Selecting the Right Chart

The following picture helps the selection of the right type of chart.

![Chart Selection][chartSelection]

[chartSelection]: ../../../images/chart_selection.png

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