One question runs through everything:
Would a wider uncertainty make this claim easier to support?
If it would, the claim is built wrongly, whatever else is true of it. The same question the other way round: could a more precise measurement have contradicted this? If nothing could, it is not coming from the reef.
Before that makes sense, one thing has to be cleared up. Data is not good or bad. A blurred photograph is a perfectly faithful record of what a blurred photograph records. Nothing about it is bad. What differs between one frame and another is not quality but how much it constrains — how wide the uncertainty is.
A wide uncertainty, honestly stated, is not a fault. It is frequently the most useful thing you can report, and nothing in this course treats a wide result as a poor one. What is a fault is a stated uncertainty that is wrong: a chart that has faded since last year, a frame that clipped, a floor measured on the easiest day and quoted as if it held for a year. Those look exactly like good numbers and misrepresent themselves, which is why you will be recording batches and intervals and versions of things that seem like they should not need recording.
With that settled, the test is a counterfactual you run on your own work. If my measurement had been less certain, would this conclusion have been easier to reach? If the answer is yes, the conclusion is not coming from the reef. It is coming from the uncertainty.
Three ways that happens, all of them the same way:
| The claim | Widening the uncertainty about... | ...increases |
|---|---|---|
| "A third of this coral is bleached" | whether a white pixel is bleached tissue or a clipped highlight | the bleached count |
| "No change was detected" | the floor, by re-capturing carelessly on Dive 3 | the chance nothing clears the floor |
| "The two visits agree" | how the methods differed, by re-scoring the older one | apparent agreement |
The second one is the one you could commit without noticing. The floor is never smaller than a threshold the table gives you, but it can be bigger: re-capture carelessly on Dive 3 and the observed difference comes out wide, and a wide floor means nothing at your station will ever clear it. Less care made "nothing detected" more likely — which is why the frame-check ratio of your Dive 3 capture is written next to the floor, so a later reader can see whether you earned it.
The observed difference can also mislead the other way. Measured on the same day, in the same light, it is the easiest repeat there is, and on its own it would be a floor far too small for a return a year later — a difference that is only the interval's noise would clear it and look like change. That is why the floor is never less than the sampling threshold, why your Entry records how long apart the two captures were, and why a real return lists what changed rather than pretending to compute it.