Retinal variables:

• Size
• Value (saturation/opacity)
• Orientation
• Texture
• Shape
• Position
• Color

Two types of retinal variables:

• An associative variable does not affect the visibility of other dimensions (e.g. we can recognize color regardless of orientation.)
  Hue, Orientation, Texture, Shape, Position are associative.


• A dissociative variable significantly affect the visibility of other dimensions (e.g. its hard to determine color of a thin line or small dot).
  Size and Value are dissociative (they dominate perception and disrupt processing of other correlated dimensions)

• Categorical: Distinct categories should be obvious. For example, different states, jobs, or students.
  Which variables can be used for categorical scale?


• Ordered: Determine relative ordering. For example, you rate your professor on the scale: good, medium, bad
  Position, size, and value are ordered


• Quantitative: Determine amount of difference between ordered values. For example, family income in US dollars.
  Position and size are quantitative

Color brewer is a good resource for color selection: http://colorbrewer2.org/