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I have a form with a field "quantity" which can have the value "unlimited / infinity":

Quantity: [ ]

What would be a good way to let users enter "infinity"?

Some ideas:

1) Tell the user that -1 means infinity:

Quantity: [ ] (-1 = unlimited)

2) Tell the user that a blank field means infinity:

Quantity: [ ] (blank = unlimited)

3) Radio buttons

() Quantity: [ ]  or  () unlimited

Any ideas? Or links to related usability studies?

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up vote 14 down vote accepted

I prefer option #3 as it does not require the user to do anything special such as entering a special number or character. This makes it more difficult for the user to make a mistake. It also is easy to handle programmatically and, assuming you name your fields and variables appropriately, make your code cleaner and easy to maintain since the special value is separated from the "normal" values.

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Thanks for your opinion and reasoning. I have chosen variant 3) now and will refine it a bit with jQuery to disable the textbox, if "unlimited" is selected. – deamon Oct 25 '11 at 11:44

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