I'm a big fan of Design Sprints, especially of their value to efficiently align a team on the problem and proposed hypothesis/solution. There's one aspect with which I've always struggled: testing.
On the last day, the team tests their idea with 5 participants. This sample size is cited as sufficient per Nielsen Norman Group's study on the sample size needed to achieve optimal usability testing results (i.e., identify most usability problems).
But is the research objective on Day 5 of a Design Sprint truly a usability test? I don't think so. The objective, to me, is more of a concept test: does this proposed solution solve your meaningful problem? (And not: is this proposed solution intuitive to use to achieve your goal for a specific real world scenario?)
Example: On the last day of the Design Sprint, say a team tests the hypothesis that a flying car can solve the need of frustration of sitting in traffic. It's testing if the flying car meets the need as expected by target customers (as opposed to a hyperloop or super sonic hedgehog). It's not testing whether using the flying car - i.e., opening the door, starting it up, managing it through the flight, etc. - has usability problems that need resolved.
Is it usually acceptable to test a concept with 5 participants? What is an ideal sample size to test a concept?