The effects of environmental characteristics on the structure of hospital clusters.

The population ecology view that variation in sets or clusters of organizations should be isomorphic with variation in cluster environment was used here to explain structural variation among hospital clusters. The structural characteristics studied were range of services offered within the cluster, average size of hospitals in the cluster, and cluster differentiation. In the causal model that was developed and evaluated, variation in the patient environment and variation in the supplier environment were compared. Four lagged panels of data on a national sample of fifteen hospital clusters demonstrated the relative superiority of supplier variables over patient variables. Supplier group preferences were more powerful than patient needs in determining the range of services offered by the cluster. Furthermore, increasing the range of services in the cluster had a positive, significant effect on average hospital size, whereas size apparently exerted no effect on range of cluster facilities. Cluster differentiation seems to be causally affected by range of services, average hospital size, and by the periodic closing of hospitals over time.