The objective of this highly successful concept is the systematic reduction of variability and the accompanying isolation of sources of difficulties
during production
The notion of using sampling and statistical analysis techniques in a production setting had its beginning in the
1920s
Purpose of a Control Chart
The purpose of a control chart is to determine if the performance of a process is maintaining an acceptable level of quality.
Control Chart
A control chart is intended as a device to detect the nonrandom or out-of-control state of a process.
Type of Errors
Type I (Process in control) Type II (Process out of control)
Type I Error (Alpha)
Occurs when looking for a special cause of variation when in reality a common cause is present.
Type 2 Error (Beta)
Occurs when assuming that a common cause of variation is present when in reality there is a special cause.
Variation
Law of nature in that no two natural items in any category are the same.
Sources of Variation
Equipmemt
Material
Environment
Operator
Two Types of Variation
Common Cause
Special Cause
Types of Data
Variable data
x-bar and R-charts
x-bar and s-charts
Charts for individuals (x-charts)
Attribute data
For “defectives” (p-chart, np-chart)
For “defects” (c-chart, u-chart)
Out-of-Contol Patterns
- Change or jump in level
- Trend or steady change in level
- Recurring cycles
- Two populations
Control Charts for Variables
Are means of visualizing the variations that occur in the central tendency and the mean of a set of observations. It shows whether or not a process is in a stable state.
Objectives of variable control charts
- For quality improvement
- To determine the process capability
- For decisons regarding product specifications
- For current decisions on the production process
- For current decisions on recently produced items
Quality Characteristic 7 Basic Units
- Length
- Mass
- Time
- Electrical current
- Temperature
- Substance
- Luminosity
Process in Control
When special causes have been eliminated from the process to the extent that the points plotted on tge control chart remain within the control limits, the process is in a state of control.
When a process is in control, there occurs a
Natural pattern of variation
State of Control: When the process is in control-
- Individual units of the product or service will be more uniform
- Since the product is more uniform, fewer samples are needed to judge the quality
- The process capability or spread of the process is easily attained from 6sigma
- Trouble can be anticipated before it occurs
- It is an indication that the operator is performing satisfactorily
