I think this is a good question - I hope someone who actually knows about this stuff chimes in with some answers. In the mean time as a thought experiment, goi

I think this is a good question - I hope someone who actually knows about this stuff chimes in with some answers. In the mean time as a thought experiment, going with a blood test example: * levels of the compound being tested may rise or fall naturally, and the test checks for a certain concentration or above. * the patient may have the condition, but not have had it long enough for markers to have risen to "trigger" levels. * The patient may not exactly follow pre-draw instructions on eating etc, skewing results * in the case of false negative: the person's immune response may be temporarily suppressing the marker * in the case of something like cell counts - this sample could just be a random local variation * The tech or doctor or whoever could randomly make a mistake on one sample, but not on each sample. And so on. My devil's advocate point here is that the patient, doctor, lab, and so on are not deterministic code - there are a lot of random inputs in the entire process chain.