Types of Variables and Distributions



Investigators must have a thorough understanding of the types of variables they might encounter as part of their research. Variables can be described using distributions. Some common distributions are described, along with specific types of variables, including independent, dependent, and confounding variables, which are defined and described. Understanding variables and distributions is imperative for planning and interpreting research studies.


Variables Distributions Parameters 


  1. 1.
    Altman DG. Statistics notes: variables and parameters. Br Med J. 1999;318:1667.  https://doi.org/10.1136/bmj.318.7199.1667.CrossRefGoogle Scholar
  2. 2.
    Larson MG. Descriptive statistics and graphical displays. Circulation. 2006;114:76–81.  https://doi.org/10.1161/CIRCULATIONAHA.105.584474.CrossRefPubMedGoogle Scholar
  3. 3.
    Booth CM, Eisenhauer EA. Progression-free survival: meaningful or simply measurable? J Clin Oncol. 2012;30(10):1030–3.  https://doi.org/10.1200/JCO.2011.38.7571.CrossRefPubMedGoogle Scholar
  4. 4.
    Saad ED, Katz A. Progression-free survival and time to progression as primary end points in advanced breast cancer: often used, sometimes loosely defined. Ann Oncol. 2008;20(3):460–4.  https://doi.org/10.1093/annonc/mdn670.CrossRefPubMedGoogle Scholar
  5. 5.
    Gourgou-Bourgade S, Cameron D, Poortmans P, et al. Guidelines for time-to-event end point definitions in breast cancer trials: results of the DATECAN initiative (Definition for the Assessment of Time-to-Event Endpoints in Cancer Trials). Ann Oncol. 2015;26(5):873–9.  https://doi.org/10.1093/annonc/mdv106.CrossRefPubMedPubMedCentralGoogle Scholar
  6. 6.
    Altman DG, Bland JM. Statistics notes: the normal distribution. Br Med J. 1995;310:298.  https://doi.org/10.1136/bmj.310.6975.298.CrossRefGoogle Scholar
  7. 7.
    Shang Y. A note on the central limit theorem for dependent random variables. ISRN Probabil Stat. 2012;2012:192427.  https://doi.org/10.5402/2012/192427.CrossRefGoogle Scholar
  8. 8.
    McDonald JH. Handbook of biological statistics. 3rd ed. Baltimore, MD: Sparky House Publishing; 2014. p. 24–8.Google Scholar
  9. 9.
    Lachin JM. Statistical properties of randomization in clinical trials. Control Clin Trials. 1988;9(4):289–311.  https://doi.org/10.1016/0197-2456(88)90045-1.CrossRefPubMedGoogle Scholar
  10. 10.
    Altman DG, Dore CJ. Randomisation and baseline comparisons in clinical trials. Lancet. 1990;335:149–53. https://doi.org/10.1016/0140-6736(90)90014-V.CrossRefGoogle Scholar
  11. 11.
    Pond GR. Statistical issues in the use of dynamic allocation methods for balancing baseline covariates. Br J Cancer. 2011;104(11):1711–5.  https://doi.org/10.1038/bjc.2011.157.CrossRefPubMedPubMedCentralGoogle Scholar
  12. 12.
    Pocock SJ, Simon R. Sequential treatment assignment with balancing for prognostic factors in the controlled clinical trial. Biometrics. 1975;31(1):103–15.  https://doi.org/10.2307/2529712.CrossRefPubMedGoogle Scholar

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© Springer International Publishing AG, part of Springer Nature 2018

Authors and Affiliations

  1. 1.McMaster UniversityHamiltonCanada

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