Open Access Open Badges Research article

A methodological approach to identify external factors for indicator-based risk adjustment illustrated by a cataract surgery register

Ursula Hahn12*, Irmingard Neuhann3, Stefanie Schmickler4 and Frank Krummenauer1

Author Affiliations

1 Institut für Medizinische Biometrie und Epidemiologie, Fakultät für Gesundheit der Universität Witten/Herdecke, Alfred-Herrhausen-Straße 50, 58448 Witten, Germany

2 OcuNet GmbH & Co KG, Friedrichstraße 47, 40217 Düsseldorf, Germany

3 Medizinisches Versorgungszentrum Prof. Neuhann Augenheilkunde Neurologie Psychiatrie, Helene-Weber-Allee 19, 80637 München, Germany

4 Augen-Zentrum-Nordwest Augenpraxis Ahaus, Domhof 15, 48683 Ahaus, Germany

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BMC Health Services Research 2014, 14:279  doi:10.1186/1472-6963-14-279

Published: 25 June 2014



Risk adjustment is crucial for comparison of outcome in medical care. Knowledge of the external factors that impact measured outcome but that cannot be influenced by the physician is a prerequisite for this adjustment. To date, a universal and reproducible method for identification of the relevant external factors has not been published. The selection of external factors in current quality assurance programmes is mainly based on expert opinion. We propose and demonstrate a methodology for identification of external factors requiring risk adjustment of outcome indicators and we apply it to a cataract surgery register.


Defined test criteria to determine the relevance for risk adjustment are “clinical relevance” and “statistical significance”. Clinical relevance of the association is presumed when observed success rates of the indicator in the presence and absence of the external factor exceed a pre-specified range of 10%. Statistical significance of the association between the external factor and outcome indicators is assessed by univariate stratification and multivariate logistic regression adjustment.

The cataract surgery register was set up as part of a German multi-centre register trial for out-patient cataract surgery in three high-volume surgical sites. A total of 14,924 patient follow-ups have been documented since 2005. Eight external factors potentially relevant for risk adjustment were related to the outcome indicators “refractive accuracy” and “visual rehabilitation” 2–5 weeks after surgery.


The clinical relevance criterion confirmed 2 (“refractive accuracy”) and 5 (“visual rehabilitation”) external factors. The significance criterion was verified in two ways. Univariate and multivariate analyses revealed almost identical external factors: 4 were related to “refractive accuracy” and 7 (6) to “visual rehabilitation”. Two (“refractive accuracy”) and 5 (“visual rehabilitation”) factors conformed to both criteria and were therefore relevant for risk adjustment.


In a practical application, the proposed method to identify relevant external factors for risk adjustment for comparison of outcome in healthcare proved to be feasible and comprehensive. The method can also be adapted to other quality assurance programmes. However, the cut-off score for clinical relevance needs to be individually assessed when applying the proposed method to other indications or indicators.

Quality assurance; Outcome quality; Confounder; External factors; Cataract surgery; Register