Annals of Biomedical Engineering

, Volume 41, Issue 2, pp 349–365 | Cite as

The Appropriate Use of Approximate Entropy and Sample Entropy with Short Data Sets

  • Jennifer M. Yentes
  • Nathaniel Hunt
  • Kendra K. Schmid
  • Jeffrey P. Kaipust
  • Denise McGrath
  • Nicholas StergiouEmail author


Approximate entropy (ApEn) and sample entropy (SampEn) are mathematical algorithms created to measure the repeatability or predictability within a time series. Both algorithms are extremely sensitive to their input parameters: m (length of the data segment being compared), r (similarity criterion), and N (length of data). There is no established consensus on parameter selection in short data sets, especially for biological data. Therefore, the purpose of this research was to examine the robustness of these two entropy algorithms by exploring the effect of changing parameter values on short data sets. Data with known theoretical entropy qualities as well as experimental data from both healthy young and older adults was utilized. Our results demonstrate that both ApEn and SampEn are extremely sensitive to parameter choices, especially for very short data sets, N ≤ 200. We suggest using N larger than 200, an m of 2 and examine several r values before selecting your parameters. Extreme caution should be used when choosing parameters for experimental studies with both algorithms. Based on our current findings, it appears that SampEn is more reliable for short data sets. SampEn was less sensitive to changes in data length and demonstrated fewer problems with relative consistency.


Step length Step width Step time Nonlinear analysis Entropy Gait 



Approximate entropy


Sample entropy


Standard error of the difference



Funding was provided by the NASA Nebraska Space Grant & EPSCoR, Patterson Fellowship through the University of Nebraska Medical Center and NIH/NIA (R01AG034995).


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Copyright information

© Biomedical Engineering Society 2012

Authors and Affiliations

  • Jennifer M. Yentes
    • 1
  • Nathaniel Hunt
    • 1
  • Kendra K. Schmid
    • 2
  • Jeffrey P. Kaipust
    • 1
  • Denise McGrath
    • 1
  • Nicholas Stergiou
    • 1
    • 3
    Email author
  1. 1.Nebraska Biomechanics Core Facility, Department of Health, Physical Education, and RecreationUniversity of Nebraska at OmahaOmahaUSA
  2. 2.Department of Biostatistics, College of Public HealthUniversity of Nebraska Medical CenterOmahaUSA
  3. 3.Department of Environmental, Agricultural & Occupational Health, College of Public HealthUniversity of Nebraska Medical CenterOmahaUSA

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