Retention uniformity, or RU, is a concept in thin layer chromatography.
The coefficient lies always in range <0,1> and 0 indicates worst case of separation (all Rf values equal to 0 or 1), value 1 indicates ideal equal-spreading of the spots, for example (0.25,0.5,0.75) for three solutes, or (0.2,0.4,0.6,0.8) for four solutes.
This coefficient was proposed as an alternative to earlier approaches, such as D (separation response), Ip (performance index) or Sm (informational entropy).
If two values are not separated, it still indicates some uniformity of chromatographic system.
For example, the Rf values (0,0.2,0.2,0.3) (two compounds not separated at 0.2 and one at the start ) result in RU equal to 0.3609.