Lecture 6 - Statistical Process Control Charts
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35 Terms
What is an I-chart?
An Individuals chart used to monitor single measurements taken one at a time.
When is an I-chart used?
When each unit can be inspected individually and subgrouping is not useful.
What does an I-chart monitor?
The actual individual value of each observation over time.
What is the center line of an I-chart?
The average of all individual observations (x̄).
How is the average moving range calculated?
MR̄ = sum of all moving ranges divided by the number of ranges.
Why is 2.66 used in I-chart calculations?
It comes from 3/d2 where d2 = 1.128.
What is an MR-chart?
A Moving Range chart used to monitor variation between consecutive observations.
What does an MR-chart monitor?
The variation or difference between one point and the previous point.
When is an MR-chart used?
Along with an I-chart to detect process variability.
What is the center line of an MR-chart?
The average moving range (MR̄).
Why is the LCL often zero for MR-charts?
Because range values cannot be negative.
What is an X-bar chart?
A control chart that monitors the average of a subgroup of samples.
When is an X-bar chart used?
When several samples are collected at each sampling point.
What does an X-bar chart monitor?
The average process performance over time.
What is the center line of an X-bar chart?
The overall average of subgroup averages (X double bar).
What is an R-chart?
A control chart that monitors the range within each subgroup.
What does an R-chart monitor?
Process variation within a subgroup.
When is an R-chart used?
Alongside an X-bar chart to check subgroup variability.
How is range calculated in an R-chart?
Maximum value minus minimum value in the subgroup.
What is the center line of an R-chart?
The average range (R̄).
How is the UCL for an R-chart calculated?
UCL = D4(R̄)
How is the LCL for an R-chart calculated?
LCL = D3(R̄)
Why are X-bar and R charts used together?
X-bar monitors the process average while R-chart monitors process variation.
When should I use I-MR charts instead of X-bar and R charts?
When only one sample is taken at a time instead of subgroups.
When should I use X-bar and R charts instead of I-MR charts?
When multiple samples are collected at each sampling point.
What does UCL mean?
Upper Control Limit, the highest acceptable variation before the process is considered out of control.
What does LCL mean?
Lower Control Limit, the lowest acceptable variation before the process is considered out of control.
What does CL mean?
Center Line, the average or expected process value.
What does an out-of-control point mean?
A point outside the control limits indicating special cause variation.
What does a point above UCL suggest?
The process may be out of control due to unusual high variation.
What does a point below LCL suggest?
The process may be out of control due to unusual low variation.
Why are control charts important?
They help detect problems early and maintain process consistency.
What does d2 represent?
A statistical constant used in I-chart calculations.
What do D3 and D4 represent?
Constants used to calculate MR-chart and R-chart control limits.
What does A2 represent?
A constant used to calculate X-bar chart control limits.