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When color is the only varied attribute, the color code is ''unidimensional''. When other attributes are varied (e.g. shape, size), the code is ''multidimensional'', where the dimensions can be ''independent'' (each encoding separate variables) or ''redundant'' (encoding the same variable). Partial redundancy sees one variable as a subset of another.<ref name="CHRIST75"/> For example, [[playing card suit]]s are multidimensional with color (black, red) and shape (club, diamond, heart, spade), which are partially redundant since clubs and spades are always black and diamonds and hearts are always red.
The ideal color scheme for a categorical color code depends on whether speed or accuracy is more important.<ref name="JONES62">{{cite journal |last1=Jones |first1=Mari Riess |title=Color Coding |journal=Human Factors: The Journal of the Human Factors and Ergonomics Society |date=December 1962 |volume=4 |issue=6 |pages=355–365 |doi=10.1177/001872086200400604}}</ref> Despite humans being able to distinguish 150 distinct colors along the hue dimension during comparative task, evidence supports that color schemes where colors differ only by hue (equal [[luminosity]] and [[colorfulness]]) should have a maximum of 8 categories with optimized stimulus spacing along the hue dimension,<ref name="JONES62"/> though this would not be color blind accessible. Adding redundant coding of luminosity and colorfulness adds information and increases speed and accuracy of color decoding tasks.<ref name="JONES62"/> Color codes are superior to others (encoding to letters, shape, size, etc.) in certain types of tasks. However, in tasks that require rapid and precise identification, numerals or line inclination may be superior to color.<ref name="JONES62"/> Adding color as a redundant attribute to a numeral or letter encoding in search tasks decreased time by 50-75%.{{r|CHRIST75|at=Fig9}}
Tasks using categorical color codes can be classified as identification tasks, where a single stimulus is shown and must be identified ([[color task|connotatively or denotatively]]), versus search tasks, where a color stimulus must be found within a field of heterogenous stimuli.<ref name="JONES62"/><ref name="CHRIST75"/> Performance in these tasks is measured by speed and/or accuracy.<ref name="CHRIST75">{{cite journal |last1=Christ |first1=Richard E. |title=Review and Analysis of Color Coding Research for Visual Displays |journal=Human Factors: The Journal of the Human Factors and Ergonomics Society |date=December 1975 |volume=17 |issue=6 |pages=542–570 |doi=10.1177/001872087501700602}}</ref>
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