Theory and methods
Potential for bias in waiting time studies: events between
enrolment and admission
B Soboleva, P Brownb, D Zeltb
a Queen's Health
Policy Research Unit, Queen's University at Kingston, Canada, b Department of Surgery, Queen's
University at Kingston
Correspondence to: Dr Sobolev, Queen's University, 3rd Floor Abramsky Hall, Kingston Ontario K7L 3N6 Canada (bs9{at}post.queensu.ca)
Accepted for publication 5 June 2001
STUDY OBJECTIVE
To
demonstrate the effect of exclusion of data on delays in scheduling
operations in calculating difference in admission rates between two
enrolment periods.
DESIGNA prospective
cohort study; outcome measurewaiting time for elective admission;
study variablesenrolment periods, before 31 March 1997 and after that
date; the time of scheduling delay; gender; age; urgency, and type of surgery.
SETTINGAn acute care
hospital in Ontario, Canada.
PARTICIPANTS1173
consecutive cases accepted for elective vascular surgery between 1 July
1994 and 31 March 1999.
MAIN RESULTSBefore
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.
CONCLUSIONSThe
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.
Keywords: elective admission; between group comparisons
© 2001 by Journal of Epidemiology and Community Health
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