Article

Comparison of statistical methods for calculating life expectancy after spinal cord injury

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Abstract

Study design

Retrospective observational.

Objectives

To compare results of several different methods for calculating life expectancy in the same sample of people with spinal cord injury (SCI), and critically assess their advantages and disadvantages.

Setting

Two spinal centres in Great Britain.

Methods

Survival status of persons with traumatic SCI injured between 1943 and 2010 with follow-up to 2015 was determined. Standardised mortality ratios (SMRs) were calculated using age at injury and current (attained) age, and compared. Life expectancy was then estimated using the SMR methods and compared with the results of a method based on multivariate logistic regression of a person-year dataset. Life expectancy estimates calculated by applying SMRs based on current age to general population period (current) and cohort (projected) life tables were also compared.

Results

The estimated life expectancies were significantly higher when the SMRs were based on age at injury. They were also higher when a general population cohort life table was used, particularly for younger ages. With the exception of the ventilator-dependent group, the life expectancy estimates derived from logistic regression were slightly lower than those derived from SMRs based on current age and a general population period life table.

Conclusions

The multivariate logistic regression of person-years method offers several advantages compared to the SMR method for calculating life expectancy after SCI, the main ones being: greater statistical power and precision with smaller sample sizes, the ability to include more predictive factors and to distinguish the otherwise confounded effects of current age, time post-injury, and calendar time.

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Acknowledgements

The study was financially supported by Buckinghamshire Healthcare NHS Trust Charitable Spinal Fund and Ann Masson Legacy for Spinal Research Fund, UK. The original part of the study [21] was supported in part by the National Institute on Disability and Rehabilitation Research Grant #H133.G90010, USA. The authors wish to thank Medical Records staff at Stoke Mandeville and Southport Spinal Centres, and especially Mrs Pauline Cato, for their help with medical notes retrieval.

Author information

Affiliations

  1. University of Alabama at Birmingham, Birmingham, Alabama, USA

    • Michael J DeVivo
  2. National Spinal Injuries Centre, Stoke Mandeville Hospital, Buckinghamshire Healthcare NHS Trust, Aylesbury, UK

    • Gordana Savic
    • , Hans L Frankel
    •  & Mohamed Ali Jamous
  3. North West Regional Spinal Injuries Centre, Southport Hospital, Southport and Ormskirk NHS Trust, Southport, UK

    • Bakulesh M Soni
  4. Craig Hospital, Englewood, Colorado, USA

    • Susan Charlifue
  5. Rehabilitation Studies Unit, The University of Sydney, NSW State Spinal Cord Injury Service, Agency for Clinical Innovation, Sydney, Australia

    • James W Middleton
  6. John Walsh Centre for Rehabilitation Research, The University of Sydney, Sydney, Australia

    • John Walsh

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Conflict of interest

The authors declare that they have no conflict of interest.

Corresponding author

Correspondence to Gordana Savic.