Ch. 6 (Statistical Process Control-SPC) Flashcards Preview

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Flashcards in Ch. 6 (Statistical Process Control-SPC) Deck (13)
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Statistical process control (SPC)

Statistical process control (SPC)

A process used to monitor standards by taking measurements and corrective action as a product or service is being produced.


Control chart

Control chart

A graphical presentation of process data over time.


Natural variations

Natural variations

Variability that affects every production process to some degree and is to be expected; also known as common cause.


Assignable variation

Assignable variation

Variation in a production process that can be traced to specific causes.


x-bar chart

x-bar chart

A quality control chart for variables that indicates when changes occur in the central tendency of a production process.




A control chart that tracks the “range” within a sample; it indicates that a gain or loss in uniformity has occurred in dispersion of a production process.


Central limit theorem

Central limit theorem

The theoretical foundation for x-bar charts, which states that regardless of the distribution of the population of all parts or services, the distribution of x-bars (averages) tends to follow a normal curve as the number of samples increases.


calculate standard deviation

1. find average of the sample means. = x-dblbar

2. UCL: add 3*(1/sqrt(N))

3. LCL: subtract 3*(1/sqrt(N))

4. Plot points


calculate standard deviation with range values

1. find average of the sample means. = x-dblbar

2. 2. UCL: add (A2 based on sample size*range mean)

3. LCL: subtract (A2 based on sample size*range mean)

4. Plot points


calculate R-chart limits

UCL: d4*R-bar
Mean: R-bar
LCL: d3*R-bar


percent defective- p-chart

1. calculate mean of defectives. p-bar

2. p-bar*(1 - p-bar)

3. divide by size of each sample (100 data in one sample)

4. sqrt root of everything

5. multiply by 3 and add to/subtract from p-bar for UCL/LCL


number of defective- c-chart

1. average of complaints. c-bar

2. add or subtract 3*sqrt(c-bar)


When to use each chart

1. x-bar or R-bar chart are used with variables

2. p-chart is a yes or no and percentage of one.

3. c-chart is the # of an occurrence