The first part of my answer is: “Make sure you sketch, first.”

At the design stage, sketching, wire-framing, and prototyping are not equal. Sketching is useful at the divergent phase of design because it lets the design participants express and capture lots of different ideas quickly and anywhere that pen and paper will work. Nothing is as fast as running a pen across a sheet of paper to capture an idea—and then another, and another. And since sketching is intentionally rough, everyone can do it.

Responding to Œ the problem statement, first  saturate the design space with lots of ideas, and then Ž analyse and rapidly iterate them to  a design solution.

I also believe sketching is great for the convergent phase of design, but there are potential hurdles that design participants may encounter. It can be challenging to convey complex interaction, 3D manipulation, transitions, and multi-state or highly interactive GUI in sketches without learning a few additional techniques. This is unfortunate, because having to learn additional techniques reduce the near-universal accessibility of sketching.

The second part of my answer, therefore, is that “if you need to learn additional techniques to make sketching work, feel free to choose wireframes or prototyping as alternatives when there are compelling reasons to do so.”

I should point out that the three techniques—sketching, wireframing, and prototyping—are not mutually exclusive. Wireframes and paper prototypes can both be sketched—especially for simple or relatively static GUI designs.

There are no validity concerns with the use of low-fidelity sketches, as these readings show:

• Low-fidelity enables rapid and extensive exploration.
• There’s no difference between high- and low-fidelity models in usability testing.
• Designers can choose whichever medium and level of fidelity suits their practical needs and design goals.