An Association Between ICP-Derived Data and Outcome in TBI Patients: The Role of Sample Size

Brenno Cabella, Joseph Donnelly, Danilo Cardim, Xiuyun Liu, Manuel Cabeleira, Peter Smielewski, Christina Haubrich, Peter J A Hutchinson, Dong Ju Kim, Marek Czosnyka

Research output: Contribution to journalArticlepeer-review

23 Citations (Scopus)


Background: Many demographic and physiological variables have been associated with TBI outcomes. However, with small sample sizes, making spurious inferences is possible. This paper explores the effect of sample sizes on statistical relationships between patient variables (both physiological and demographic) and outcome. Methods: Data from head-injured patients with monitored arterial blood pressure, intracranial pressure (ICP) and outcome assessed at 6 months were included in this retrospective analysis. A univariate logistic regression analysis was performed to obtain the odds ratio for unfavorable outcome. Three different dichotomizations between favorable and unfavorable outcomes were considered. A bootstrap method was implemented to estimate the minimum sample sizes needed to obtain reliable association between physiological and demographic variables with outcome. Results: In a univariate analysis with dichotomized outcome, samples sizes should be generally larger than 100 for reproducible results. Pressure reactivity index, ICP, and ICP slow waves offered the strongest relationship with outcome. Relatively small sample sizes may overestimate effect sizes or even produce conflicting results. Conclusion: Low power tests, generally achieved with small sample sizes, may produce misleading conclusions, especially when they are based only on p values and the dichotomized criteria of rejecting/not-rejecting the null hypothesis. We recommend reporting confidence intervals and effect sizes in a more complete and contextualized data analysis.

Original languageEnglish
Pages (from-to)103-107
Number of pages5
JournalNeurocritical Care
Issue number1
Publication statusPublished - 2017 Aug 1

Bibliographical note

Funding Information:
Many thanks to Addenbrooke’s Hospital Neurocritical Care and University of Cambridge Department of Anaesthesia Staff for long-standing support in computerized bedside monitoring of TBI patients. This work was supported by National Institutes of Health Research (Cambridge Centre) and NIHR Health Technology Co-operative. B.C. is supported by a CNPQ Scholarship (Research Project 203792/2014-9). J.D. is supported by a Woolf Fisher Trust Scholarship. D.C. is supported by a Cambridge Commonwealth European and International Trust Scholarship. X.L. is supported by a Gates Cambridge Trust Scholarship.

Publisher Copyright:
© 2016, The Author(s).


  • Autoregulation
  • Intracranial pressure
  • Outcome prediction
  • Statistical inference
  • Traumatic brain injury

ASJC Scopus subject areas

  • Critical Care and Intensive Care Medicine
  • Clinical Neurology


Dive into the research topics of 'An Association Between ICP-Derived Data and Outcome in TBI Patients: The Role of Sample Size'. Together they form a unique fingerprint.

Cite this