Abstract
Systems for collecting information about patient care are increasingly common in orthopaedic practice. Databases can allow various comparisons to be made over time. Significant decisions regarding service delivery and clinical practice may be made based on their results.
We set out to determine the number of cases needed for comparison of Thirty-day mortality, inpatient wound infection rates and mean hospital length of stay, with a power of 80% for the demonstration of an effect at a significance level of p< 0.05. We analysed two years of prospectively collected data on 1,050 hip fracture patients admitted to a city teaching hospital. Power calculations were performed using standard equations from the literature.
Detection of a 10% difference in 30 day mortality would require 14,065 patients in each arm of any comparison, demonstration of a 50% difference would require 643 patients in each arm; For wound infections, demonstration of a 10% difference in incidence would require 23,921 patients in each arm and 1127 patients for demonstration of a 50% difference; For length of stay, a difference of 10% would require 1,479 patients and 6660 patients for a 50% difference.
Our data helps to illustrate the importance of sample size when interpreting the results of performance monitoring. Since a unit as large as our own only admits 525 patients per year, comparisons between trusts would require very prolonged data collection. Within a single unit the demonstration of poor performance by one surgeon or one team would clearly be impossible. Statistically meaningful analyses will only be possible with major multi-centre collaborations, as will be possible if hospital Trusts participate in the National Hip Fracture Database.
Correspondence should be addressed to Editorial Secretary Mr ML Costa or Assistant Editorial Secretary Mr B.J. Ollivere at BOA, 35–43 Lincoln’s Inn Fields, London WC2A 3PE, England; Email: mattcosta@hotmail.com or ben@ollivere.co.uk
