Endgames Statistical Question

Cox proportional hazards regression

BMJ 2013; 347 doi: http://dx.doi.org/10.1136/bmj.f4919 (Published 09 August 2013) Cite this as: BMJ 2013;347:f4919
  1. Philip Sedgwick, reader in medical statistics and medical education
  1. 1Centre for Medical and Healthcare Education, St George’s, University of London, London, UK
  1. p.sedgwick{at}sgul.ac.uk

Researchers measured the effect on one year mortality of secondary drug prevention for patients with stroke in routine primary care. They used a cohort study design, which incorporated patient data from the health improvement network primary care database. Participants were 12 830 patients aged 50 years or more from 113 general practices. They had all had a stroke between 1995 and 2005 and survived the first 30 days after the stroke. Secondary drug prevention was defined as being prescribed either antihypertensives plus lipid lowering drugs plus antithrombotics or antihypertensives plus lipid lowering drugs.1

Cox proportional hazards regression was used to investigate one year mortality, defined as death from any cause from 31 days after the stroke and within the first year. Univariable and multivariable analyses between one year mortality and secondary drug prevention, sex, socioeconomic deprivation, and age group were performed (table). Socioeconomic deprivation was measured by the Townsend score, which assesses socioeconomic deprivation in families and includes measurement of employment status, overcrowding, car ownership, and owner occupation status.

View this table:

Hazard ratios for association between one year mortality and secondary drug prevention, sex, Townsend deprivation index, and age group in patients after stroke

On average, mortality within the first year was 5.7% for patients receiving secondary drug prevention compared with 11.1% for patients not receiving treatment. Secondary drug prevention was associated with a 50% reduction in mortality risk (adjusted hazard ratio 0.50, 95% confidence interval 0.42 to 0.59).

Which of the following statements, if any, are true?

  • a) The outcome variable for the Cox proportional hazards regression was continuous

  • b) The hazard ratio predicts the relative proportions of patients who will have died in the categories of each variable at the end of follow-up

  • c) It was assumed that for each category of the explanatory variables the hazard of death …

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