title: Potential for bias in waiting time studies: events between enrolment and admission.
abstract: OBJECTIVE
  To demonstrate the effect of exclusion of data on delays in scheduling operations in calculating difference in admission rates between two enrolment periods.
  
  
  METHODS
  A prospective cohort study; outcome measure-waiting time for elective admission; study variables-enrolment periods, before 31 March 1997 and after that date; the time of scheduling delay; gender; age; urgency, and type of surgery.
  
  
  METHODS
  An acute care hospital in Ontario, Canada.
  
  
  METHODS
  1173 consecutive cases accepted for elective vascular surgery between 1 July 1994 and 31 March 1999.
  
  
  RESULTS
  Before adjustment for scheduling delays, a 20% lower admission rate was associated with period 2, rate ratio (RR) = 0.8 (95% confidence intervals (CI)= 0.7, 0.9). The difference between the periods became only marginally significant after the adjustment, RR = 0.9 (95% CI=0.8, 1.0). No difference between the periods was found when admission rates were compared before a delay occurred, RR = 0.9 (95% CI=0.8, 1.1). In delayed patients, those enrolled in period 1 and 2 had, respectively, a 40% and a 60% lower admission rate than the period 1 patients admitted without scheduling delays, RR = 0.6 (95% CI=0.4, 0.8) for period 1 and RR = 0.4 (95%CI=0.3, 0.5) for period 2.
  
  
  CONCLUSIONS
  The results provide evidence that patients experiencing a delay in scheduling operation have a lower admission rate after the event. Thus, potential for bias exists when between group comparison of waiting time is done without adjustment for an intermediate event that may occur before elective admission.
authors.affiliation: Queen's Health Policy Research Unit, Queen's University at Kingston, Canada. bs9@post.queensu.ca
authors.forename: B
authors.initials: B
authors.lastname: Sobolev
authors.forename: P
authors.initials: P
authors.lastname: Brown
authors.forename: D
authors.initials: D
authors.lastname: Zelt
mesh_terms: D000328:Adult; D000368:Aged; D015982:Bias; D015331:Cohort Studies; D017558:Elective Surgical Procedures / Q000706:statistics & numerical data*; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009864:Ontario; D010343:Patient Admission / Q000706:statistics & numerical data*; D011446:Prospective Studies; D013997:Time Factors; D014656:Vascular Surgical Procedures / Q000706:statistics & numerical data*; D014850:Waiting Lists