Run Charts

1
Run Charts

• Measurements for a process characteristic are
  plotted in time order
• Patterns in the data indicate special causes of
  variability
• Trends
• Clusters
• A repeating pattern
• No variability in plotted points

2
Control Charts

• A Run Chart with statistically calculated limits
• Limits are based on data collected from the
  process
• If points plot within the limits and show no
  patterns, the process is said to be "in control"
  or stable
• If the process is considered stable, limits
  represent the expected range of variation for the
  value plotted
• A process that is in control is predictable (it
  may or may not be producing desirable output)

3
Setting Up Control Charts

• Step 1 Determine question(s) to be answered
• Step 2 Design data collection plan and collect
  data
• Step 3 Plot run chart and look for obvious
  patterns
• Step 4 If no patterns, calculate control limits
  (using formulas for the appropriate type of
  chart)
• Step 5 Conduct runs tests
• Step 6 Interpret the chart
• Step 7 Determine appropriate type of action and
  take steps to accomplish this

4
Control ChartsAttributes Data

• p or np charts
• "n" items are studied
• each item is classified in one of two categories
• we are counting the number in one of the
  categories
• a p chart plots the proportion in one category
• an np chart plots the number in one category
• c or u charts
• an inspection unit (IU) is defined
• the number of occurrences are counted and plotted

5
Control ChartsVariables Data

• X-bar and R charts (used together)
• X-bar (the average of n observations) attempts to
  assess location
• R (the range of n observations) attempts to
  assess spread
• X and Moving Range charts (used together)
• When there is no logical grouping, individual
  values are plotted on the X chart
• A moving range is used to assess spread

6
Determine characteristic to study
Counting or Measuring
counting
measuring
Attributes Data
Variables Data
Classifying into two categories?
How manyitems persubgroup?
no
yes
one
2 to 8
Constant subgroup size?
Constant area of opportunity?
ConsiderX/mR
yes
no
yes
no
Considernp or p
Considerp
Considerc
Consideru
7
Situations for Study

• Insurance Company Quote Lines
• Time to make quotes
• Nursing Home Meal Delivery Process
• Timeliness
• Complaints
• Mental Health Facility's Use of 11s
• Number of orders per week

8
Run Charts

• If the process is stable all of the following
  will be true
• Most points will plot near some central value
• Some variation will exist
• Individual points will not be predictable, but
  the overall clustering of points will be
  predictable
• No patterns will show up

9
Runs Tests-Melton

• A control chart fails to show stabilityif any of
  the following occur
• at least one point plots outside the control
  limits
• two of three consecutive points in the same A
  zone
• fifteen consecutive points plot in the C zones
• more than seven consecutive points on the same
  side of the center line
• seven or more consecutive increases (or
  decreases)
• fifteen consecutive points alternating up and
  down

Note Other runs tests are available in other
books.
10
LCL 6.386 CL 15.467UCL 24.547