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P chart & c-chart
1. P-CHART & C-CHART
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2. STATISTICAL PROCESS CONTROL
• It involves monitoring the production process
to detect and prevent poor quality.
• It is a statistical procedure using control
charts to see if any part of a production
process is not functioning properly and could
cause poor quality.
• It is a tool for identifying problems in order to
make improvement.
4. • ATTRIBUTE: An attribute is a product
characteristics such as colour , surface
texture,cleanlines, smell and taste.
• Attribute can be evaluated quickly with a
discrete response such as good or bad.
• If quality specification are complex and
extensive , a simple attribute test might be
used to determine whether or not a product
or service is defective.
6. Spc applied to
• Hospitals
• Grocery store
• Airlines
• Fast food restuarant
7. Control charts
• A graph that establishes the control limits of a
process.
• These are graphs that visually show if a
sample is within statistical control limits.
• Two basic purpose:
1. to establish the control limits for a process.
2. To monitor the process to indicate when it is
out of control.
8. Statistical control charts
• It is one of graph to monitor a production
process.
• Samples are taken from the process periodically ,
and observation are plotted on the graph .
• If any observation is outside the upperlimit or
lower limit on the graph, it indicate that
something is wrong in the process.
• ie, it is not in control which may cause defective
or poor quality items.
9. Where control charts used?
• Control charts are used at critical points in the
process where historically the process has
shown a tendency to go out of control .
• At points where if the process goes out of
control it is particularly harmful and costly.
• It is frequently used at the beginning of a
process to check the quality of raw materials
and delivers for a service operation.
10. • Used before a costly or irreversible point in
process
• After which the product is difficult to rework
• Before or after assembly or painting
operations that might cover defects.
• Before the outgoing final product or service is
shipped or delivered
11. Types of the control charts
• Variables control charts
– Variable data are measured on a continuous scale. For example: time,
weight, distance or temperature can be measured in fractions or
decimals.
– Applied to data with continuous distribution
Eg: X chart and R chart
• Attributes control charts
– Attribute data are counted and cannot have fractions or decimals.
Attribute data arise when you are determining only the presence or
absence of something: success or failure, accept or reject, correct or
not correct. For example, a report can have four errors or five errors,
but it cannot have four and a half errors.
– Applied to data following discrete distribution
Eg: P chart and C chart
(http://www.asq.org/learn-about-quality/data-collection-analysis-tools/overview/control-chart.html)
12. Two types :
• P-chart & C-chart for attributes
• Mean X and range R for variables
13. P chart
• Also called the percent defective chart
• Uses the proportion of defective items in a
sample as the sample statistic.
• P-chart can be used when it is possible to
distinguish between defective and non
defective items and to state the number of
defectives as a percentage of the whole.
16. • UCL= UPPER CONTROL LIMIT
• LCL= LOWER CONTROL LIMIT
• Z= the no. of standard deviations from the
process average.=3
• P= process % defective of a sample
• P bar =process mean percent defective
19. C -Chart
• Also called the number of defective per
sample area.
• It applies to the no. of nonconformities in
sample of constant size
• C=no. of nonconformities in each sample.
• The CL of this chart are based on poisson
distribution.
20. Application of c chart
• To control the no. of nonconforming rivets in
an aircraft wing.
• To control the number of imperfection
observed in a galvanized sheet
• To control the no. of defects in final
assemblies(like TV, radio, computer)