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.