A 24-Point Drop Is Not a 24% Saving
Ada re-runs the frost-protection saving and keeps percentage points and proportional reduction apart
ADA · CALCULATION AUDIT
A 24-Point Drop Is Not a 24% Saving
A citrus grower’s frost-protection system cut false wind-machine activations from 40% to 16% of frost events, and with 30 machines burning 30 needless hours a season at $85 per hour the baseline waste is $76,500. The tempting read is to subtract the rates — 40 minus 16 — and bank a 24% saving. This audit re-runs the figure and asks whether the value tracks the 24-point gap or the 60% proportional reduction, a difference of $27,540.
Companion to the chapter Smart Agriculture and Livestock — every number here comes from that chapter.
The frost example says false wind-machine activations fell “from 40% to 16% of frost events,” then asks for the fuel saving. The trap is to subtract — 40 minus 16 is 24 points, so surely you save 24% of the cost. Let me keep the percentage-point drop and the proportional saving apart, because only one of them scales the money.
The baseline waste
30 wind machines running 30 unnecessary hours a season at $85 per hour:
The reduction — two different numbers
From 40% to 16% is a drop of 24 percentage points:
But as a fraction of the original false-alarm rate it is a 60% proportional reduction:
The saving follows the proportion, not the point gap:
What the trap would have paid
Subtracting the points instead would pay out:
— barely 40% of the real figure, and short by:
Design meaning: when a rate improves from one percentage to another, the value it unlocks tracks the relative reduction between them, not the difference in points — conflating the two is how an IoT business case quietly mis-states its own payback.
Every number above is taken from the chapter’s own frost-protection example and re-derived step by step.