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Nature and extent of selection bias resulting from convenience sampling in the emergency department
  1. Travis Lines1,
  2. Christine Burdick2,
  3. Xanthea Dewez2,
  4. Emogene Aldridge3,
  5. Tom Neal-Williams4,
  6. Kimberly Walker5,
  7. Hamed Akhlaghi2,6,
  8. Buntine Paul3,
  9. David McDonald Taylor6,7
  1. 1Alfred Health, Prahran, Victoria, Australia
  2. 2Department of Emergency Medicine, St Vincent's Hospital Melbourne Pty Ltd, Fitzroy, Victoria, Australia
  3. 3Emergency Medicine, Eastern Health, Melbourne, Victoria, Australia
  4. 4Northern Health, Epping, Victoria, Australia
  5. 5Western Health, Footscray, Victoria, Australia, Footscray, Victoria, Australia
  6. 6Department of Medicine, The University of Melbourne, Melbourne, Victoria, Australia
  7. 7Emergency, Austin Health, Heidelberg, Victoria, Australia
  1. Correspondence to Professor David McDonald Taylor, Emergency, Austin Health, Heidelberg, VIC 3084, Australia; dmcdtaylor{at}


Background To compare the clinical and demographic variables of patients who present to the ED at different times of the day in order to determine the nature and extent of potential selection bias inherent in convenience sampling

Methods We undertook a retrospective, observational study of data routinely collected in five EDs in 2019. Adult patients (aged ≥18 years) who presented with abdominal or chest pain, headache or dyspnoea were enrolled. For each patient group, the discharge diagnoses (primary outcome) of patients who presented during the day (08:00–15:59), evening (16:00-23:59), and night (00:00-07:59) were compared. Demographics, triage category and pain score, and initial vital signs were also compared.

Results 2500 patients were enrolled in each of the four patient groups. For patients with abdominal pain, the diagnoses differed significantly across the time periods (p<0.001) with greater proportions of unspecified/unknown cause diagnoses in the evening (47.4%) compared with the morning (41.7%). For patients with chest pain, heart rate differed (p<0.001) with a mean rate higher in the evening (80 beats/minute) than at night (76). For patients with headache, mean patient age differed (p=0.004) with a greater age in the daytime (46 years) than the evening (41). For patients with dyspnoea, discharge diagnoses differed (p<0.001). Asthma diagnoses were more common at night (12.6%) than during the daytime (7.5%). For patients with dyspnoea, there were also differences in gender distribution (p=0.003), age (p<0.001) and respiratory rates (p=0.003) across the time periods. For each patient group, the departure status differed across the time periods (p<0.001).

Conclusion Patients with abdominal or chest pain, headache or dyspnoea differ in a range of clinical and demographic variables depending upon their time of presentation. These differences may potentially introduce selection bias impacting upon the internal validity of a study if convenience sampling of patients is undertaken.

  • methods
  • emergency department

Data availability statement

Data are available on reasonable request. no further detail.

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Data availability statement

Data are available on reasonable request. no further detail.

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  • Handling editor Richard Body

  • Contributors DMT and TL conceived the study. DMT, TL, HA and BP developed the study protocol and obtained ethics committee approval. TL, CB, XD, EA, TN-W and KW collected the data. DMT, HA and BP supervised the data collection. DMT collated, cleaned and undertook the data analysis. DMT, TL, HA and BP interpreted the results. All authors contributed to writing the manuscript and approved the final, submitted manuscript. DT is the author acting as guarantor.

  • Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.

  • Competing interests None declared.

  • Provenance and peer review Not commissioned; externally peer reviewed.

  • Supplemental material This content has been supplied by the author(s). It has not been vetted by BMJ Publishing Group Limited (BMJ) and may not have been peer-reviewed. Any opinions or recommendations discussed are solely those of the author(s) and are not endorsed by BMJ. BMJ disclaims all liability and responsibility arising from any reliance placed on the content. Where the content includes any translated material, BMJ does not warrant the accuracy and reliability of the translations (including but not limited to local regulations, clinical guidelines, terminology, drug names and drug dosages), and is not responsible for any error and/or omissions arising from translation and adaptation or otherwise.