You are right. The alert systems can only be designed with two options and the third condition (TN) is always inherent in the system design. I also meant the same but probably a wrong use of word “should” in the above sentence led to confusion. My example of TN vs FN was more to drive home the point of Type 2 error(False Negative) being more dangerous than Type 1 error (False positive). I agree there is room for confusion in my current article as it is hinting at possibility of TN and FN design as well. Anyways I will be updating this article with the focus on calculating other ratios without being confused. In the process I will reword sentences which is causing the misunderstanding. Thanks for your feedback :)