This question wasn't part of my survey, but it was part of another survey. I thought the results were relevant enough to include here as part of this series.

A 2013 survey by Programmable Web that included about 250 respondents found that complete and accurate documentation was the most important factor in an API:

This is certainly an encouraging finding for technical writers, and suggests that more attention and focus should be given to documentation. I think I'm going to print this picture out and tape it on my office wall.

Why is documentation so important for APIs in contrast to GUI-based products? API documentation is important because it is the interface for the product. With APIs, there is no user interface that people can figure out on their own. If you don't have clear and accurate documentation, the developer won't figure out how to use the API. Can you imagine guessing what an endpoint might be, or what parameters it takes?