Form a team of knowledgeable individuals with respect to this quality issue.    The team will select a number, e.g., from six to twelve, of the potential causes from the Cause & Effect diagram.   Call these potential causes concerns.   The process for generating the Interrelationship Digraph will construct causal relationships among the concerns.   The word digraph is a combination of the two words diagram and graph.  The resulting digraph reflects the collective judgment of the team.

Benbow and Kubiak (2005, page 40) specify a procedure for constructing the digraph.   List the concerns on a sheet of easel paper or a whiteboard. Pick a pair of concerns.   Ask the team to specify whether the first concern influences the second, the second concern influences the first, or whether there is no influential relationship between the concerns.    If the team decides there is an influential relationship, draw an arrow from the most influential concern to the other concern.  Does the first concern influence the second more than the second concern influences the first?   If so, draw an arrow from the first concern to the second.  Repeat this assessment for all possible pairs.  A good way to proceed is to arrange the concerns in an approximate circular pattern.   Start with the concern in the 12 o’clock position and call it the first concern.   Compare it with the concern in the next clockwise position.  Then, move clockwise and select another concern to compare with the first concern.   Repeat this process until all possible combinations of concerns have been compared by the team.

The next posting will illustrate the construction of an interrelationship digraph.

This posting gives an example of an Interrelationship Digraph which is a tool for use in the seventh step, Study Cause and Effect, of the Hoerl-Snee Process Improvement Strategy.   The quality issue is the potential causes or factors contributing to late deliveries.   We take our example from Benbow and Kubiak (2005).   The interrelationship digraph appears below.

1. ‘Poor scheduling practices’ (6 outgoing arrows),
   
2. ‘Late order from customer’ (5 outgoing arrows), and
   
3. ‘Equipment breakdown (3 outgoing arrows).
   

A concern with a large number of input arrows is affected by a large number of other concerns.  Thus, it could be a source of a quality or performance metric.   ‘Poor scheduling of the trucker’ has 4 input arrows.   A measure of poor scheduling performance of the trucker could indicate the magnitude of system problems causing late delivery.

References:

1. Benbow, D. W. and T. M. Kubiak (2005). The Certified Six Sigma Black Belt Handbook. Milwaukee, Wisconsin, ASQ Quality Press.

GOAL/QPC, an educational consulting company, noticed a new book proposing seven new QC tools.   This book (Mizuno, 1988) was eventually translated into English.  GOAL/QPC created the Memory Jogger Plus+ (Brassard, 1989) featuring these new tools.  They called these new tools the ‘Seven Management and Planning Tools’ to differentiate them from the ‘Seven Major SPC Tools’.   The Seven Management and Planning Tools are:

1. Affinity diagram
2. Interrelationship digraph
3. Tree diagram
4. Prioritization matrices
5. Matrix diagram
6. Process decision program chart (PDPC)
7. Activity network diagram

Montgomery (2005, page 148) identifies ‘Seven Major SPC Tools’.  He calls them the ‘Magnificent Seven’.  They are:

1. Histogram (3/25/2008 and 5/1/2008 postings) or stem-and-leaf plot
2. Check sheet
3. Pareto chart (2/25/2008 and 5/18/2008 postings)
4. Cause and effect diagram (2/28/2008 posting)
5. Defect concentration diagram
6. Scatter diagram (3/28/2008 posting)
7. Control chart (1/30/2008, 2/11/2008, 2/14/2008, and 4/1/2008 postings)

Earlier, Ishikawa (1985) identified ‘Seven Major TQM’ (Total Quality Management) tools.   They are:

1. Histogram
2. Flowchart
3. Pareto chart
4. Cause and effect diagram
5. Run charts and graphs
6. Scatter diagram
7. X-bar and R control charts

One could say that Montgomery replaced the ‘flowchart’ and ‘run charts and graphs’ with the ‘check sheet’ and ‘defect concentration diagram’.   Montgomery also generalized the X-bar and R control charts with all control charts.

References

1. Brassard, M. (1989). The Memory Jogger Plus+^â. Salem, NH, Goal/QPC.
2. Mizuno, S. (1988). Management for Quality Improvement: The Seven New QC Tools. Cambridge, Productivity Press.