Figure 1.

States and trajectories in physiological state space. Hypothetical plot shows values of three physiological variables measured in different individuals at different times. Measurements define clusters of points close in Euclidean distance in this space, here denoted by colored circles. Identification of such clusters is a quantitative means allowing researchers to define, operationally, qualitatively distinct physiological states. In the example here, the different colors correspond to states qualitatively used to describe health and sickness, and arrows indicate transitions between different states observed in different individuals. Values of variables from a given individual over time define a trajectory in this space.