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You may have also noticed the increased wait time to generate more-thorough scenarios. Not only is the number of tests getting larger, but any margin of error in your System under Test decreases. As you may have guessed, this tradeoff between test quantity and quality continues all the way up to 4-, 5-, and even 6-way interactions. This is because tests will include every quadruplet, quintuplet, and sextuplet of test conditions, respectively.

• 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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