Suppose green mould is suspected in a new compost batch. Two readings are available: compost pH and room light. Only one changes the decision. If pH is below 7.2 the recipe or Phase II must be corrected; if it sits inside 7.2 to 7.8 the search moves to spawn and casing. Light changes nothing.
That is the test for every measurement: write the decision first, then ask whether each possible result leads to a different action. ICAR-DMR's compost chapter gives the splitting numbers for button compost: pH 7.2 to 7.8, moisture 68 to 72 percent for synthetic long-method compost or 64 to 66 percent after Phase II, and ammonia below 10 ppm at spawning.