The four parts every result comes out of
Every result comes out of a machine with four parts, and each question in this volume is a question about one of those parts.
Every result you are shown comes out of the same machine, and the machine has four parts. Naming them is worth one page, because after this the eight questions stop being a list to memorize and become a map of one object.
| the part | what it is | what can go wrong with it |
|---|---|---|
| the cases | somebody chose a set of situations to test | the wrong ones, too few of them, or ones the thing had already seen |
| the run | the cases were put through some version, on some day | a version that no longer exists, and runs that never finished |
| the grading | something ruled on each answer | nobody knows what ruled, and nothing ever checked it |
| the number | the rulings were added up and one figure came out | nothing, usually. This is the part that is almost always correct |
The last row is the surprising one. The arithmetic is nearly always right. Almost every bad result you will ever be handed is a correct sum over the wrong cases, or a correct sum whose losses happened earlier in the machine. Doubting the number is the wrong instinct and it makes you look unserious. Ask about the other three parts.
This is also how to handle a question this volume never asked. Something about a result bothers you and you cannot name it: work out which of the four parts your unease is about, and the question usually writes itself.