Precision Is Not Accuracy: How Much to Trust a Calculated Number
Software supplies unlimited precision at no cost, which makes it very easy to produce a figure that is exact and wrong.
Blog
Short, practical pieces about the arithmetic behind the tools — the conversion errors that cost money, the statistics that mislead, and how much to trust a calculated number.
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Why Every Calculator Here Runs in Your Browser
There is no technical reason for a mortgage calculator to send your salary anywhere. Disconnect the internet and check.
Dates Are Harder Than They Look
From the 1st to the 8th is seven days apart and spans eight. Nearly every date argument starts there.
Pricing a Product: Markup, Margin and the Fees Nobody Models
A price is not revenue. Between the two sit fees most spreadsheets either omit or model the wrong way round.
The Weighted Average Is Everywhere and Almost Nobody Sees It
Never average a set of ratios. Recover the numerators, recover the denominators, add each, divide once.
Reading a Health Calculator Honestly
BMI was designed in the 1830s to describe populations. The ideal-weight formulas were built for drug dosing. Neither was meant for you.
What Your Timesheet Is Actually Costing You
Forty-six hours on site and 43.25 paid is a six per cent gap that never appears on a payslip. Here is where it goes.
Compound Interest Is Not Intuitive, and That Is the Whole Point
Human intuition about repeated multiplication is wrong by orders of magnitude, consistently and in the same direction.
Why Your Percentage Is Wrong: Points, Per Cent and the Missing Denominator
Almost every misleading percentage has the same defect: a ratio detached from its denominator. Four ways it goes wrong.
The 27 Cubic Feet Problem: Unit Errors That Cost Real Money
A cubic yard is 27 cubic feet, not 9. Four conversion errors that produce plausible-looking numbers and expensive mistakes.