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Statistical Inference by Confidence Intervals: Issues of Interpretation and Utilization

J Sim, PhD, PT, Professor, Department of Physiotherapy Studies, Keele University, Keele, Staffordshire ST5 5BG, United Kingdom (pta05{at}keele.ac.uk). Address all correspondence to Dr Sim
N Reid, DPhil, Professor of Health Sciences, Office of the Vice-Chancellor, University of Plymouth, Plymouth, Devon, United Kingdom

This article examines the role of the confidence interval (CI) in statistical inference and its advantages over conventional hypothesis testing, particularly when data are applied in the context of clinical practice. A CI provides a range of population values with which a sample statistic is consistent at a given level of confidence (usually 95%). Conventional hypothesis testing serves to either reject or retain a null hypothesis. A CI, while also functioning as a hypothesis test, provides additional information on the variability of an observed sample statistic (ie, its precision) and on its probable relationship to the value of this statistic in the population from which the sample was drawn (ie, its accuracy). Thus, the CI focuses attention on the magnitude and the probability of a treatment or other effect. It thereby assists in determining the clinical usefulness and importance of, as well as the statistical significance of, findings. The CI is appropriate for both parametric and nonparametric analyses and for both individual studies and aggregated data in meta-analyses. It is recommended that, when inferential statistical analysis is performed, CIs should accompany point estimates and conventional hypothesis tests wherever possible.

Key Words: Confidence intervals • Estimation • Hypothesis testing • Statistical inference

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