One tool that can be used for quality control is a control chart. Control Chart can be used to determine whether there has been a production process changes, detect the presence of the causes that affect the process, and create a standardized process. Variations of product quality often arise. In the production process, there are two types of variations that arise in the production process. Namely :
1. Assignable cause
Assignable cause is the variation that can be sought sources of the cause, and it should be removed. For example, the quality of raw materials are not homogeneous, the instructions are less obvious, less good mechanical condition.
2. Random (common cause)
Common cause is a natural variation (variation that is not traceable to their sources), and variations of this type can not be eliminated 100%, but minimized. One example is the quality of raw materials are made as homogeneous as possible, pursued the same level of operator skill.
Control Chart can be divided into two types, Attribute Control Chart and Variables Control Chart. Quality characteristics that can be measured and expressed quantitatively named variables, while the quality is assessed as appropriate or inappropriate (defects) are called attributes. Control chart provides information about the ability of the process, an important process parameter values, and stability over time so as to provide estimates of process capability. This information is very useful for designing products and processes.
Understanding of the attributes in quality control related to quality characteristics that can be classified on the good (acceptable) and disabled (turned down). Some kind of Control Chart for Attributes are :
1. p chart
control chart for the fraction defective (rejected fraction fraction nonconforming).
2. np chart
control chart for the number of items that are not appropriate (number of nonconforming).
3. c chart
control chart for the number of mismatches (number of nonconformities)
4. u chart
control chart for the number of nonconformities per unit (number of nonconformities per unit).
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