1B Practical arithmetic
Chapter 1 · pages 22–25 · 843 words
This section sets out the small arithmetic habits that make hand analysis of data manageable. First, precision is a matter of judgment rather than a fixed rule: sometimes you keep extra decimal places, sometimes you round, depending on how the rest of the data behaves. If nearby values are close together, more precision matters; if they are far away, a rounded answer loses nothing.
When rounding is done, ties (numbers ending in exactly .5) are broken by rounding to the nearest even number rather than always up. So 17.5 becomes 18, but 18.5 also becomes 18, while 19.5 becomes 20. This differs from cutting, where you simply drop the extra digits without adjusting the remaining ones — cutting is faster but less accurate. The choice between the two is pragmatic: cut when more detail is available nearby if you need it later, round when the data matters enough that you want a fair, careful value.
For writing numbers down, a single number is clearest when both its raw units and decimal point are kept (like 0.00127 inches). But once several numbers must be compared, decimal points become a nuisance because they disrupt visual alignment and make comparison harder. The fix is to choose convenient units so the numbers become clean integers — expressing population in hundreds of thousands rather than in millions with decimals, for instance — and to label the table or panel with the unit used, such as "unit = 100,000 people," so the reader knows how to interpret the bare digits.
Finally, the section introduces small notational shortcuts used throughout the book: "h" stands for ".5" or "and a half" (so counting by halves goes 4, 4h, 5, 5h, 6...), **"*" is a place-holder digit used when cutting a number to indicate unknown or insignificant trailing digits (always by cutting, not rounding), and "#"** is shorthand for "count" or "number." These conventions let numbers be written compactly without pretending to a precision the data doesn't have.
flowchart LR
A[Raw number] --> B{Need to compare many numbers?}
B -- No, single number --> C[Keep units and decimal point]
B -- Yes --> D[Pick convenient units]
D --> E[Drop decimal points]
E --> F[Label panel with unit]
Exhibits and original pages (3)
