Article Text

Download PDFPDF
Mortality, readmission and length of stay have different relationships using hospital-level versus patient-level data: an example of the ecological fallacy affecting hospital performance indicators
  1. Stefanie N Hofstede1,
  2. Leti van Bodegom-Vos1,
  3. Dionne S Kringos2,3,
  4. Ewout Steyerberg4,5,
  5. Perla J Marang-van de Mheen1
  1. 1 Department of Medical Decision Making, Leiden University Medical Center, Leiden, The Netherlands
  2. 2 Department of Public Health, AMC, Amsterdam, The Netherlands
  3. 3 Amsterdam Public Health Research Institute, University of Amsterdam, Amsterdam, The Netherlands
  4. 4 Department of Public Health, Erasmus MC, Rotterdam, The Netherlands
  5. 5 Department of Medical Statistics and Bioinformatics, Leids Universitair Medisch Centrum, Leiden, Zuid-Holland, The Netherlands
  1. Correspondence to Dr Perla J Marang-van de Mheen, Leiden University Medical Centre, Leiden, 2300 RC, Netherlands; p.j.marang{at}


Background Ecological fallacy refers to an erroneous inference about individuals on the basis of findings for the group to which those individuals belong. Suppose analysis of a large database shows that hospitals with a high proportion of long length of stay (LOS) patients also have higher than average in-hospital mortality. This may prompt efforts to reduce mortality among patients with long LOS. But patients with long LOS may not be the ones at higher risk of death. It may be that hospitals with higher mortality (regardless of LOS) also have more long LOS patients—either because of quality problems on both counts or because of unaccounted differences in case mix. To provide more insight how the ecological fallacy influences the evaluation of hospital performance indicators, we assessed whether hospital-level associations between in-hospital mortality, readmission and long LOS reflect patient-level associations.

Methods Patient admissions from the Dutch National Medical Registration (2007–2012) for specific diseases (stroke, colorectal carcinoma, heart failure, acute myocardial infarction and hip/knee replacements in patients with osteoarthritis) were analysed, as well as all admissions. Logistic regression analysis was used to assess patient-level associations. Pearson correlation coefficients were used to quantify hospital-level associations.

Results Overall, we observed 2.2% in-hospital mortality, 8.1% readmissions and a mean LOS of 5.9 days among 8 478 884 admissions in 95 hospitals. Of the 10 disease-specific associations tested, 2 were reversed at hospital-level, 3 were consistent and 5 were only significant at either hospital-level or patient-level. A reversed association was found for stroke: patients with long LOS had 58% lower in-hospital mortality (OR 0.42 (95% CI 0.40 to 0.44)), whereas the hospital-level association was reversed (r=0.30, p<0.01). Similar negative patient-level associations were found for each hospital, but LOS varied across hospitals, thereby resulting in a positive hospital-level association. A similar effect was found for long LOS and readmission in patients with heart failure.

Conclusions Hospital-level associations did not reflect the same patient-level associations in 7 of 10 associations, and were even reversed in 2 associations. Ecological fallacy thus potentially influences interpretation of hospital performance when patient-level associations are not taken into account.

  • mortality (standardized Mortality Ratios)
  • quality measurement
  • healthcare quality improvement
  • continuous quality improvement

Statistics from

Request Permissions

If you wish to reuse any or all of this article please use the link below which will take you to the Copyright Clearance Center’s RightsLink service. You will be able to get a quick price and instant permission to reuse the content in many different ways.


  • Contributors PJM-vM designed the study. SNH wrote the article and carried out the study. PJM-vM supervised the study and writing of the manuscript. All authors have critically read and modified both the study protocol and previous drafts of the manuscript and have approved the final version. All authors read and approved the final manuscript.

  • Funding This research project is supported by a grant from the Netherlands Organisation for Health Research and Development (grant number 516022513).

  • Competing interests None declared.

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