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> That fits with both personal experience and what I've read, but I'd
> the rule more than Mark has done: the difference should typically be at
> 20% (probably the source of the 10 points + 20% = 12 points that Mark used
> an example). This will depend on the font and size to some extent, but
> still a reasonably robust rule of thumb. It's also got some
> with George Miller's old research on perception that led to the "rule of 7
> or minus 2". In many scales, humans can only easily distinguish between 7
> discrete steps at a time, and if you opt for a larger margin of safety
> 5) you get 5 steps. On a percentage scale, that would be 100/5=20%.
> This may simply be me committing psychobabble, or it may be a legitimate
> extension of Miller's research. But I've found in other areas (e.g.,
> shades of grey for use in labeling graphs), a difference of at least 20%
> required for the difference to be easily detectable.
I don't think this has anything to do with Miller's cognitive studies, which
focused on memory, not on perception. Rather, it has to do with the Just
Noticeable Difference (JND), a common measure in psychology. The JND varies
a good deal depending on many factors, including the intensity of the
stimulus. The lower the stimulus, the smaller the JND. The JND also varies a
lot depending on which senses are involved. Your example of gray shading on
graphs is just such an example of the JND. You might find that the shades
needed varied according to the pixel density and how much area the gray
covered on the page.
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