Physiotherapy March 2000/vol 86/no 3

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AbstractsAdvancing Physiotherapy Practice

The Physiotherapy Research Society held a scientific meeting at the University of Birmingham on October 21,1998, at which these abstracts were presented. Publication was delayed by a technical difficulty.

The next meeting will be held at the University of Brighton (Eastbourne campus) on Thursday, April 6, 2000. Applications to attend that meeting or inquiries about joining the Physiotherapy Research Society

should be made to Mrs Lynne Caladine, University of Brighton, Robert Dodd Building, 49 Darley Road, Eastbourne BN20 9AY.

Estimation as an AlternativeApproach to Statistical Inferencein Physiotherapy Research

Julius Sim Department of Physiotherapy Studies, Keele UniversityStatistical inference in physiotherapy research is customarily bymeans of conventional hypothesis tests. This paper will examinethe role of statistical estimation using confidence intervals (Cls).Actual and hypothetical data will be used to illustrate the meritsof the CI, and in particular its advantages over the conventionalhypothesis test.

A sample statistic such as a mean is a point estimate of apopulation parameter. A Cl, in contrast, is an interval estimate,and embraces a range of population values with which a samplestatistic is consistent, at a given level of confidence, eg 95%(Gore, 1981). A conventional hypothesis test returns a P value,which can be used to reject or retain a null hypothesis. A Cl canalso fulfill this role, but has additional merits that thecalculation of P values does not share.

First, the width of a Cl reflects sample size and thereby givesan indication of the precision of a sample statistic as an estimateof a population parameter, which is not provided by a P valuealone.

Secondly, the level of confidence chosen for a Cl indicates the accuracy of a sample statistic, ie its probable relationship to the population parameter. A conventional hypothesis testmerely provides a binary outcome, depending on whether the P value generated is above or below a specified cut-off value (eg P --0.05), and is not informative as to the accuracy of theeffect for which it has been calculated.

Thirdly, judgments as to the clinical significance of anobserved effect are facilitated by the inspection of a Cl, whichallows the magnitude as well as the probability of such an effectto be assessed. For example, possible effects that are compatiblewith rejection of a null hypothesis, yet of little clinicalimportance, can be readily identified.

Fourthly, Cls allow the comparative evaluation of the datafrom individual studies in the context of meta-analysis.

In conclusion, it is recommended that Cls should normally begiven alongside point estimates of sample statistics wheninferences to a corresponding population parameter are to bemade. Similarly, conventional hypothesis tests shouldaccompanied by, or even replaced by, confidence intervals.

Reference

Gore, S (1981). ‘Assessing methods -- confidence intervals’,British Medical Journal, 283, 660-662.

Invited Speakers

Advancing Physiotherapy PracticeAnne Parry Sheffield Hallam University

Advancing Practice throughResearch – The challengeCollette Clifford University of Birmingham

ODFS – What is it?Christine Singleton University of Birmingham

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