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The âdefense in depthâ concept requires countermeasures to be independent and placed in âAâ and for the given examples result in âBâ likelihood of providing protection. %Assume âCountermeasure 1â is correct 70% of the time and âCountermeasure 2â is correct 90% of the time.
The protection is placed in âseriesâ (A) and results in successful countermeasure 63% (B) of the time.
The protection is placed in âparallelâ (A) and results in successful countermeasure 37% (B) of the time.
The protection is placed in âparallelâ (A) and results in successful countermeasure 3% (B) of the time.
The protection is placed in âseriesâ (A) and results in successful countermeasure 97% (B) of the time.
When placed in series only one of the countermeasures needs to work for success. Therefore the likelihood of success is 1 minus both fail (or 1-0.3 * 0.1 which is 1-0.03 which is 0.97). If they were in parallel then they both must work so the answer is 63% (or 0.9 * 0.7 = 0.63). 97% success is better than 63% success so you place in series.
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