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We've been debating this question with a client. In a table which displays percentages, should the percent sign % be displayed once in the header of the column Header (%) or in each of the cells 80 %.
There was a 50/50 split in opinions without real and concrete arguments on which is better for the user. Does anyone has argument for either of the options?
If it's a single column, it's debatable and depends on how clearly you label the header, and what sort of data is in neighbouring columns for example, but I would generally advise including the % symbol for clarity.
However, if it affects the whole table, and the table is more than a few rows or columns then the visual clutter would be overwhelming, for example a BMI chart:
Other options to explore might include reducing the visual impact of the percentage signs using a little whitespace, lighter font, or reduced font sizes, all of which help the numerical value stand out better.
The header in tables is a complementary element that provides a contextual help to what's in the actual columns - but it's the data itself that interest users. Ideally, only new users will use the header at the first few usage instances (unless the columns are very similar in nature, in which case the header will be used much more often - I don't know what's in the columns in your case, so it's hard to pass judgement).
The argument for the sign being only in the header is that doing so reduces repetition and visual noise. Yet, users can do the 1 + 1 (column value + header) to understand the semantics (but this does mean calculated meaning - users have to work out two elements to infer meaning).
However, in the case of percentages, I suspect that a plain number can be confusing - most numbers represent a magnitude, whereas a percent is rational. I think this is a distinction worth making explicit.
I suspect, that you'd get more complaints if you omit the % sign from the rows than if you wouldn't.
Personally, I feel that in this specific case the visual noise added is worth it, as you gain making something rather distinctive explicit.
