Sketches dominate the early ideation stages, whereas prototypes are more concentrated at the later stages where things are converging within the design funnel. Much of this has to do with how they differ with respect to the attributes of cost, timeliness, quantity, and disposability that we discussed above as characterizing sketches.
Essentially, the investment in a prototype is larger than that in a sketch, hence there are fewer of them, they are less disposable, and they take longer to build. At the front-end of the funnel, when there are lots of different concepts to explore, and things are still quite uncertain, sketching dominates the process.
They are not prototypes. While both sketches and prototypes are instantiations of a design concept, they serve different purposes, and therefore are concentrated at different stages of the design process.
The relevant attributes of sketches are: