Table 1.

Description of entropy measures used in concussion populations and their limitations.

Entropy measure	Description	Limitation
Approximate entropy	Calculated by determining the probability that if two series data points are similar for a length of m points, then they will remain similar at the next point	Biased toward regularity in that it includes self-matches and lacks relative consistency in that it is more sensitive to changes in input parameters, m and r
Sample entropy	A modification of approximate entropy that does not include self-matches and is independent of data length	Does not discriminate between groups with a shorter data as well as approximate entropy
Shannon entropy	Calculated by mapping a CoP trace onto a grid and then determining the probability of the CoP occupying a given box during the CoP trace	Substantial influence of samples on Shannon entropy
Renyi entropy	A mathematical generalization of Shannon entropy, which does not assume the additivity of independent events	Substantial influence of samples on Renyi entropy

Abbreviation: CoP = center of pressure.