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6.5 MECE - Mutually Exclusive

The first aspect of the MECE rule is that within any group, the reasons and objections should be mutually exclusive - that is, they should be genuinely distinct from each other.

 
Discussion
   
Because the two objections make basically the same point, you don't need both of them on the map.  If you get rid of the second objection (Nasa faked the evidence) then the reason for it is added to the reasons for the first objection.

This is what the above argument map looks like after corrections have been made:

The "mutually exclusive" rule does not mean that there can never be any conceptual overlap within a group.  Sometimes terms or concepts may appear in more than one reason or objection.  In fact, given that the Rabbit Rule should be observed with regard to each and every reason and objection in the group, there should be some conceptual overlap.  

It does mean that reasons and objections should be genuinely distinct from each other, even if they have some concepts in common. Informally, they should not be making the same point, wholly or even partially. 

It can be quite difficult to spot when considerations are not mutually exclusive.  It is particularly hard to notice when there are also level problems, i.e., the Pyramid rule has not been properly observed. For example

Superficially, the two objections here are really quite different; that is, they appear to be mutually exclusive.  Notice however that there is a level problem; the two objections violate the second part of the Pyramid Rule, which says that considerations at the same level should be at roughly the same level of generality or abstraction.  The second objection belongs at a lower level. 

Once the level problem is fixed, the similarity between the two objections becomes more apparent.  They are not mutually exclusive; the second objection is really part of the evidence for the first one.

 
New Concepts

Mutually exclusive (ME): Within a group, considerations should be genuinely distinct from each other. ME is the first aspect of the MECE rule.

The MECE Rule: Considerations in a group should be mutually exclusive (no overlaps) and collectively exhaustive (no gaps).

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 Last updated 28-Nov-2006