# Environmental Efficiency Indices in the Public Hospital Sector: A Proposal
## 2 Methodology
### Methodology
#### 2.2.5 Calculation of the Environmental Efficiency Index (EEI)  
Hospitals resemble companies, which in this case produce a healthcare service. The processes are different for each patient, so there are countless products. For this reason, measuring hospital production is complicated and difficult. Traditionally, hospital production was measured by the number of beds or stays in a hospital. Nowadays, technological evolution in the healthcare environment, which seeks to reduce complications and adverse events in patients, reduces stays by promoting major outpatient surgery and high resolution.  
In 1998 in Spain, there was a change at the National Health Institute (INSALUD, Spanish acronym) regarding the so-called management contracts, a new tool with which hospitals are financed based on the complexity of the processes addressed in the hospitalisation area, as well as on the different hospital products in the other production areas. The hospital complexity unit (HCU) appears in management contracts as a measure of healthcare activity. It is established using the average weight of each patient multiplied by the number of patients received annually. The mean weight is a classification system of patients of the same class who consume a similar amount of resources. The diagnosis-related group (DRG) is an American system with worldwide acceptance that establishes a relative weight for each class of patients based on their cost. Thus, for example, the weight corresponding to DRG 165 (Uncomplicated appendicitis) is 1.3502, and that of DRG 103 (Heart transplant) is 37.91.  
The process is no longer about measuring productivity according to the number of stays, but is based on complexity. To obtain the HCUs, DRG weights are applied to hospital discharges, generating an average weight for each hospital and reflecting their complexity index. The production of the centre expressed in HCUs [9] is obtained by multiplying the discharges by this index. Note that HCU = number of discharges per average weight.  
The HCU, in addition to facilitating activity information, allows the financing of the hospital activity to be known if a HCU fee is established, as it considers the two main aspects of hospital production: the complexity of the processes attended (average DRG weight per discharges) and the activity volume (number of discharges) [10].  
Therefore, a hospital's HCU value, in addition to revealing information on the hospitalisation activity, is the expression of the patients' resource consumption as they pass through the healthcare process. The HCU indicator is a dimensionless unit of measurement of hospitalisation activity that considers the relative complexity of the pathology of hospitalised patients.  
In this study, to obtain the environmental efficiency index, both for the endpoint global impact (calculated with ReCiPe) and for, in each impact category studied (CML-IA) (Equation (1)), the environmental impact calculated based on the hospital complexity unit (HCU).  
\[\text{EEI}=\frac{\text{I}}{\text{HCU}} \tag{1}\]  
where:  
I = calculated environmental impact;  
HCU = annual hospital complexity unit (annual activity).