In statistics, an approximate entropy (ApEn) is a technique used to quantify the amount of regularity and the unpredictability of fluctuations over time-series data. For example, there are two series of data: series 1: (10,20,10,20,10,20,10,20,10,20,10,20...), which alternates 10 and 20.series 2: (10,10,20,10,20,20,20,10,10,20,10,20,20...), which has either a value of 10 or 20, chosen randomly, each with probability 1/2.