You have budget for fifteen usability sessions on one product this quarter, and you can spend it however you like. What does the Nielsen Norman Group's advice point to?

The principle — a research budget buys iterations, and iterations are worth more than participants.

Why the key is correct — the finding is that The best results come from testing no more than 5 users and running as many small tests as you can afford. and the advice attached to it is explicit about how to spend money: it is better to distribute your budget for user testing across many small tests instead of blowing everything on a single, elaborate study Three rounds of five lets each round test a design that the previous round changed, which is where the learning actually happens.

Why the others are wrong — one study of fifteen buys a single detailed snapshot of a design nobody has yet had the chance to fix, and leaves nothing to check the fix with. Testing five and stopping keeps the number and throws away the part that makes it work. And two rounds moves in the right direction while leaving a third of the budget unspent on exactly what it was meant for.

Remember this — spread the budget across rounds; a bigger single study buys detail you cannot act on.

Sources — Nielsen Norman Group on testing with five users.