Medical Statistics

Diegeler et al. [NEJM 2013; 368: 1189-98] undertook a study of off-pump coronary artery bypass graft surgery in older patients. You should read this paper carefully before attempting to answer the following questions. Students are expected to answer all questions in a concise manner and marks will be awarded for clear and succinct answers

  • What type of study design is described here? (2 marks)


  • What type of randomisation has been used? Include details of any stratification if it has been used. (2 marks)


  • Why was the sample size increased after the interim analysis? (3 marks)


  • Using Table 1, what proportion of patients undergoing off-pump CABG had a history of myocardial infarction? (1 mark)


  • The analysis was performed on a modified intention-to-treat basis (ITT) (with all randomly assigned patients who underwent isolated CABG by the assigned surgeon included). Why is ITT preferable in comparison to a per-protocol analysis? (3 marks)


  • Details are given in Figure 1 on the numbers randomised and the numbers excluded after randomisation and at different stages of the trial. Use the information provided to construct a 2×2 table and perform a chi-squared test to evaluate whether there are any differences between allocated groups in the proportions of randomised patients included in the 30-day analysis. (8 marks)


  • Examine Table 1 and answer the following questions
  1. How are the logistic euroSCORE and the Koronarchirurgie score summarised? (2 marks)
  2. Comment on whether these are appropriate summary statistics to use. (2 marks)
  3. What would be an alternative way of summarising these scores (2 marks)


  • The components that make up the primary outcome were measured at 30 days (expressed as an odds ratio using logistic regression) and 12 months (Kaplan-Meier curves and log rank test). What is the primary endpoint and why do you think the authors chose different statistical tests at these time points? (6 marks)


  • Using the results from table 3;
  1. What are the odds of repeat revascularization (component of primary outcome) at 30 days for off-pump CABG patients compared to those who received on-pump CABG and how this is interpreted? (3 marks)
  2. Was the risk (hazard) of repeat revascularization different at 12 months? Justify your answer (3 marks)


  • Using the Kaplan-Meier curve from figure 2, estimate the proportion of patients in each group that are stillevent free at 180 days. (3 marks)



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