FORMS: When to use dropdowns, and when to use dynamic lists?

Interesting that I couldn't find a discussion of this already.

When should we use dropdown selects, and when should we use dynamic lists (updated as you type) in web forms?

My opinion:

Dropdowns are better when:

• you don't have too many items to choose from (30 or less?)


• your audience is not very familiar with the choices of your dropdown, and would thereby benefit from being able to look at all the possibilities

Dynamic lists are better when:

• you have a LOT of items and having to display them all//scroll through to find what you need would suck (ex: country selectors)


• you know your audience is well-familiar with all (or nearly all) possible choices and can reliably enter it themselves (ex: country or city selectors)

Other reasons // situations to choose one over the other?