Control Charts for Variable and Attribute
There are two important control charts in statistical process control (SPC) such as control chart for variable and control chart for attribute.
On the other hand, attribute control chart is used to identify the defective/conforming and nondefective/nonconforming of a unit of product which these two classifications are called quality characteristics. In the process, we may examine a unit of product and count defects or nonconformities on the unit. Then, these attributes data show the number of defective or defects per sample, or proportion of the defective or defects per sample. Control limits are calculated by many ways depending on kind of control charts. Control chart for attribute include: control chart for fraction nonconforming(p chart); control chart number of nonconforming items(np chart); control chart for nonconformities(c chart); control chart for nonconformities per unit(u chart).
Before we
understand about variable chart, we must understand what the variable is.
Variable is the single measurable quality characteristic, such as dimension,
weight, and volume etcVariable
control chart is the simple and powerful tool for determining the process
whether it is in the control or out of control by looking at the
plot points. If they stay out of upper-control limit or lower control limit, it
means that the process is out of control. Control charts help you focus
problem-solving efforts by distinguishing between common and special-cause variation.
Then, we can improve on that point in order to make the process to be in the
control. In variable control
chart, we normally monitor with x bar control chart,
which is mean quality level, s control
chart, which is a control chart for the standard deviation,
and R control chart, which is a control chart for range. The x bar and R (or s)
charts are among the most important and useful on-line statistical process
monitoring and control techniques.
On the other hand, attribute control chart is used to identify the defective/conforming and nondefective/nonconforming of a unit of product which these two classifications are called quality characteristics. In the process, we may examine a unit of product and count defects or nonconformities on the unit. Then, these attributes data show the number of defective or defects per sample, or proportion of the defective or defects per sample. Control limits are calculated by many ways depending on kind of control charts. Control chart for attribute include: control chart for fraction nonconforming(p chart); control chart number of nonconforming items(np chart); control chart for nonconformities(c chart); control chart for nonconformities per unit(u chart).
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