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 (x̄ R) | Variables | Independent | < 10, but usually 3 to 5 | Normal | Large (≥1.5σ) |
| Average and Standard Deviation (x̄ s) | Variables | Independent | Usually ≥ 10 | Normal | Large (≥1.5σ) |
| Median and Range (x~ R) | Variables | Independent | < 10, but usually 3 to 5 | Normal | Large (≥1.5σ) |
| Individuals and Moving Range (I MR) | Variables | Independent | 1 | Normal | Large (≥1.5σ) |
| Fraction Defective (p) | Attributes | Independent | Variable usually ≥ 50 | Binomial | Large (≥1.5σ) |
| Number Defective (np) | Attributes | Independent | Constant usually ≥ 50 | Binomial | Large (≥1.5σ) |
| Number of Defects per Unit (u) | Attributes | Independent | Variable | Poisson | Large (≥1.5σ) |
| Number of Defects (c) | Attributes | Independent | Constant | Poisson | Large (≥1.5σ) |
Table: Chart type properties
Selecting the Right Chart#
The following picture helps the selection of the right type of chart.
Available Pages#
Dive into the pages below to explore more about Chart Types: