You don't know the actual probability of it being accurate. You apply the test 100 times. And let's say out of those 100 times, you get that they are accurate, you get that it is accurate and you're able to use some other test that you know, some for sure test, some super accurate test to verify your own test results and you see that it is accurate 95 out of the 100 times. So the question you have is, well, does the hypothesis make sense to you? Will you accept this hypothesis? Well, what you say is, well, if my hypothesis was true, if my test were accurate 99, if the probability of my test being accurate is 99%, what's the probability of me getting this outcome? Well, we figured that out.