B.            Statistical process control (SPC)

                                                       1.            Objectives and benefits
Describe the objectives and benefits of SPC, including controlling process performance, identifying special and common causes, etc. (Analyze)

                                                       2.            Rational subgrouping
Define and describe how rational subgrouping is used. (Understand)

                                                       3.            Selection and application of control charts
Identify, select, construct, and apply the following types of control charts: −R, −s, individuals and moving range (ImR / XmR), median ()−s, individuals and moving range (ImR / XmR), median (), p, np, c, and u. (Apply)

                                                      4.            Analysis of control charts
Interpret control charts and distinguish between common and special causes using rules for determining statistical control. (Analyze)

Statistical process control(SPC) – a technique for applying statistical analysis to m measure, monitor, and control processes.

Objectives – to determine process capability, monitor processes and identify whether the process is operating as expected or whether the process has changed and corrective action is required. Control chart information can be used to determine the natural range of the process and to compare it with the specified tolerance range.

·          The risk of charting many parameters is the operator will spend so much time and effort completing the charts, that the actual process becomes secondary.

Benefits – include the ability to monitor a stable process and identify if changes occur, due to factors other than random variation.

·          Ability to detect rends of statistical significance

·          Provide an ongoing measure of process capability; ability to monitor continuous improvement efforts.

·          Detect special causes of variation

Special cause – assignable, non-normal. A source of quality failure that lies outside the process, and so is intermittent, unpredictable, unstable. Special cause variation is caused by known factors that result in a non-random distribution of output. Special cause variation is a shift in output caused by a specific factor such as environmental conditions or process input parameters. It can be accounted for directly and potentially removed and is a measure of process control. With a system under control, special causes have been eliminated. If a process is “out of control,” then special causes of variations are present in either the average chart or range chart, or both. A process out of control is detected on a chart by either having any point outside the control limits or by unnatural patterns of variability.

Common cause – chance, normal, random. A source of quality failure that is always present as part of the random variation inherent in the process itself. Its origin can usually be traced to an element of the process which only management can correct. The less well-defined a process is, the more it is subject to random variation, resulting in a higher level of quality failures. In general, and very approximately, common causes outweigh special causes as origins of quality failures by four to 1 (Pareto distribution).

Rational subgrouping - A rational subgroup is a subset of data defined by a specific factor such as a stratifying factor or a time period. Rational subgrouping identifies and separates special cause variation (variation between subgroups) caused by specific, identifiable factors.

·          Order of production subgrouping

o         All product produced as nearly as possible as one time

o         Product intended to be representative of all the production over a given period of time

o         Groups that achieve opportunity for variation from one subgroup to another

·          Consider possible sources of variation

o         Process variation

o         Stream-to-stream

o         Lot-to-lot variation

o         Time-to-time variation

o         Piece-to-piece variation

o         Within piece variation

o         Measurement variation

o         Inherent process variation

Control charts – are used to distinguish between random variation, and variation due to an out of control condition.

·          Control limits – boundaries set by the process which indicates process stability and variability. Control limits are 3 standard deviations above and below the grand average.

·          Control charts for variables – plots specific measurement of a process characteristic (temperature, size, weight, sales volume, shipments, etc.)

o         Xbar – R charts

§          100 data values considered sufficient to accurately calculate the upper and lower control limits.

§          Normal distribution

§          Xbar chart – monitors the process mean

§          R chart – monitor the process variance

o         Xbar – S charts

o         Xbar – MR charts

·          Control charts for attributes – plots general measurement of the total process (the number of complaints per order, number of orders on time, absenteeism frequency, etc.)

o          p charts

§          fraction (percentage) defective; binomial distribution

o          np charts

§          number of defects; binomial distribution

o          c charts

§          number of defects; Poisson distribution

o          u charts

§          number of defects per unit; Poisson distribution

Analysis of control charts

·          Out-of-control conditions – 5 common rules

·          1 point outside the 3 sigma limits

·          a trend of 8 points in an upward or downward direction

·          8 consecutive points on 1 side of the center line

·          4 of 5 consecutive points beyond 1 sigma

·          2 of 3 consecutive points beyond 2 sigma.

Control plans – a document describing the critical to quality characteristics, the critical Xs or Ys of the part or process. The control plan should not be a replacement for detailed operator instructions or standard operating procedures. It is a “living document,” is always subject to change, and must be kept current.

·          Types of control plans

o         Prototype

o         Pre-launch

o         Production