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Reliability, measurement error and minimum detectable change in mobility measures: a cohort study of community-dwelling adults aged 50 years and over in Ireland
  1. Orna A Donoghue1,
  2. George M Savva2,
  3. Axel Börsch-Supan3,
  4. Rose Anne Kenny1,4
  1. 1 The Irish Longitudinal Study on Ageing (TILDA), University of Dublin Trinity College, Dublin, Ireland
  2. 2 Quadram Institute Bioscience, Norwich, UK
  3. 3 Munich Center for the Economics of Aging, Max Planck Institute for Social Law and Social Policy, Munchen, Germany
  4. 4 Mercer's Institute for Successful Ageing, St. James' Hospital, Dublin, Ireland
  1. Correspondence to Dr Orna A Donoghue; odonogh{at}


Objective To estimate the effects of repeat assessments, rater and time of day on mobility measures and to estimate their variation between and within participants in a population-based sample of Irish adults aged ≥50 years.

Design Test–retest study in a population representative sample.

Setting Academic health assessment centre of The Irish Longitudinal Study on Ageing (TILDA).

Participants 128 community-dwelling adults from the Survey for Health, Ageing and Retirement in Europe (SHARE) Ireland study who agreed to take part in the SHARE-Ireland/TILDA collaboration.

Interventions Not applicable.

Outcome measures Participants performed timed up-and-go (TUG), repeated chair stands (RCS) and walking speed tests administered by one of two raters. Repeat assessments were conducted 1–4 months later. Participants were randomised with respect to a change in time (morning, afternoon) and whether the rater was changed between assessments. Within and between-participant variance for each measure was estimated using mixed-effects models. Intraclass correlation (ICC), SE of measurement and minimum detectable change (MDC) were reported.

Results Average performance did not vary between baseline and repeat assessments in any test, except RCS. The rater significantly affected performance on all tests except one, but time of day did not. Reliability varied from ICC=0.66 (RCS) to ICC=0.88 (usual gait speed). MDC was 2.08 s for TUG, 4.52 s for RCS and ranged from 19.49 to 34.73 cm/s for walking speed tests. There was no evidence for lower reliability of gait parameters with increasing time between assessments.

Conclusions Reliability varied for each test when measurements are obtained over 1–4 months with most variation due to rater effects. Usual and motor dual task gait speed demonstrated highest reliability.

  • repeatability
  • physical performance tests
  • longitudinal change
  • epidemiology

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  • Contributors Substantial contributions to the conception or design of the work; or the acquisition, analysis or interpretation of data for the work: OD, GMS, AB-S and RAK. Drafting the work or revising it critically for important intellectual content: OD, GMS, AB-S and RAK. Final approval of the version to be published: OD, GMS, AB-S and RAK. Agreement to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved: OD, GMS, AB-S and RAK.

  • Funding TILDA received financial support from the Irish Government (Department of Health and Children), the Atlantic Philanthropies and Irish Life plc. The SHARE-TILDA project was funded by the National Institute of Aging (Prime Award Number R21AG040387).

  • Competing interests None declared.

  • Patient consent for publication Not required.

  • Ethics approval Ethical approval for this substudy was obtained from the Faculty of Health Sciences Research Ethics Committee at Trinity College Dublin.

  • Provenance and peer review Not commissioned; externally peer reviewed.

  • Data availability statement TILDA considers applications for privileged access to the dataset through an onsite “hot desk” facility based in TILDA (visit for further information).

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