I'm mainly looking for referrals to studies that were objective in evaluating the effectiveness of a series of symbols, but open to opinions based on anecdotal experience too.
Symbol characteristics that influence the usability of a symbol or icon include:
- Concreteness: concrete (as opposed to abstract) symbols can depict objects that are familiar and visually obvious which facilitates recognition
- Visual complexity: simple (versus complex) symbols have shorter search and processing times; on the other hand extra detail can improve efficacy as well
- Meaningfulness: meaningful symbols factilitate the ease with which mappings may be made between symbol and function
- Familiarity: user performance improves as a result of learning
- Semantic distance: how close is the relationship between the symbol and what it is meant to represent?
Other characteristics are defined in relation to other symbols and cannot be measured in isolation, such as discriminality, distinctiveness and configurality.
See: McDougall SJ, Curry MB, & de Bruijn O. Measuring symbol and icon characteristics: norms for concreteness, complexity, meaningfulness, familiarity, and semantic distance for 239 symbols. Behavior research methods, instruments, & computers : a journal of the Psychonomic Society, Inc, 31(3), 487-519.
I'd say a symbol is effective if it immediately conveys the appropriate meaning, even out of context. Groups of symbols should be consistent in style.
You'll certainly want to find out if there are commonly used symbols for the thing you are representing before creating new ones.
If you are creating new symbols, remember:
- Common symbols sometimes differ based on location/culture.
- Colors sometimes have different meanings based on location/culture.
Anecdotal experience: a prominent symbol that was used by a North American company failed miserably when used in the Indian market because that symbol in the Indian market was similar to the logo of the family planning commmission. (They give you advice on preventing std's, using contraception, give you free contraception, etc.) Imagine you open a swank commercial office and dock workers start queuing up at your gate to get free condoms!
My point is: do your homework before you adopt a global symbol, icon, etc.
An easily recognizable or well designed symbol is great, but even better when accompanied by some sort of label or description. This can help you at first if the symbol is at all ambiguous and in the future you will find yourself only looking at the symbol rather than the text. It's the best of both worlds if you have the real estate.