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• 2-way (pairwise) coverage strength

  is often a good default strength for many types of functional tests
  
  

• 3-way coverage strength

   overall, could be considered in higher-risk / more “mission-critical” testing. If you are executing tests by hand, rather than execute a complete set of 3-way scenarios, it is often preferable to execute a smaller set of “Mixed-Strength” tests (with 3-way coverage focused on the most important interactions and 2-way coverage for the other parts).
  
  

• 4-way and higher coverage strengths

   are rarely used except in situations where (a) all the tests are automated (therefore cheap and quick to execute) and/or (b) the potential risk for error could be extremely disastrous or expensive. For instance, safely sending people into space requires that no errors be made.

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