Combo boxes offer the user a choice of two or more mutually exclusive options. Their disadvantage compared to related controls like radio buttons or lists is that the options are not visible without further interaction.
- Use combo boxes instead of radio buttons for more than four mutually exclusive options.
- Don't use them for less than three options or more than 10 choices. In the latter case, use lists instead.
- In general, controls that show the options without further user interaction are preferable, except for the following cases:
- the list of options may change over time,
- the contents are obvious from the label and the one selected item, for example Month and January
- the combo box is part of a related sequence of controls. For example, to set a reminder to ring 5 hours or minutes before or after an event.
- Provide a descriptive label to the left (in left-to-right scripts), in some cases above the combo box. Don't forget to create a buddy relation so access keys are assigned.
- Don't initiate an action when the user selects an item from a drop-down list.
- Make sure the items are easily accessible via keyboard by moving distinctive letters to the beginning of each option. For example, in a list of countries on continents, write Germany (Europe) instead of Europe/Germany.
- Use sentence style capitalization for the label and the options.
- If activating a choice affects the appearance or the enabled state of other controls, place them next to the radio button or in the next row (indented).
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