Chapter Four · The Craft
Test Design: Thinking in Cases
The heart of the book and the heart of the profession. A date field takes millions of values and you'll test seven — this chapter is the craft of choosing which seven, five named techniques deep: partitions that make most values safe to skip, boundaries where the bugs actually live, decision tables that corner interacting rules, state diagrams that expose forbidden moves and missing arrows, and the pairwise-plus-catalog pair that handles everything the others can't. Every technique taught on the Fernway rules you already know.
Principle 2 left a question hanging back in Chapter 1: if exhaustive testing is impossible, which tests do you choose? This chapter is the profession's answer, and it's the most examined skill in every junior QA interview — "how would you test this field?" is really "show me you choose deliberately," and after these six topics you will.
Watch what the chapter does to one plain sentence from the Fernway canon. "A stay is 1 to 30 nights" becomes: five partitions, boundary tests at 0, 1, 30, and 31, a seven-value designed set you can defend value by value. "SUMMER15 doesn't combine with the member discount" becomes an eight-column decision table with one cell nobody can fill — and that cell becomes a five-minute conversation with Petra instead of a production refund. The booking lifecycle becomes a state diagram with a missing arrow, and the missing arrow becomes FW-356, found before a single test ran. Plain sentences in, precise aimed checks out. That transformation is the craft.