In a complex data entry app I am trying to minimise the amount of stuff on screen. So I considered combining text entry and drop down into a single element. So the user can either select from the dropdown or type a new entry.
Does anyone have any research regarding this type of functionality? Does it work for the user? Is it intuitive?
An example of how I envisaged it: http://michaelapproved.com/articles/textbox-and-dropdown-combined
What you are describing is a Combobox and has been in use for as long as I can think about GUIs. The concept has gained new momentum in the web with the further development of dynamic elements and web apps – most notably google's search box with it's suggestions-as-you-type.
Chrome's "Omnibar" is, basically, the same thing: It's a text field you can type in anything you want, but it will also suggest stuff you previously entered.
So, yes, I think it is save to say that the basic concept is working for users.
The cases above, however, do not have any "data entry" aspect. Whatever you type into the omnibar or into google's search field is temporary data that's only used once. Thus it's usually no problem that there's lots and lots of entries, because only very few will be used over and over again.
If your application tries to establish a norm for data input I would highly suggest using a more strict type of input. The very nature of "enter anything you want" will result in people entering anything they want, and therefore lead to lots and lots of (probably redundant) entries.
So, in short:
The basic principle works
If it's a good choice depends strongly on the details of your use case
I can't say I know of any research into this, but for me this would be a no go. It just isn't intuitive enough for the user.
Perhaps a better solution would be to present the user with a drop-down in the first instance, if then the user cannot select an appropriate option, reveal an input field giving the user the ability to manually enter the relevant details.
Another way to tackle this might be to only display an input field, but have a drop down list appear beneath the input field (as the user begins to type) with suggestions as to what might be appropriate. This then allows for both options that are and aren't available within the menu, without asking the user to take any additional steps.
This isn't answering your question with research and what not but how about something like this?
http://jqueryui.com/autocomplete/#combobox
Similar but filters down and could also accept the users unique input.
