A nurse rostering system using constraint programming and redundant modeling.

This paper describes the design and implementation of a constraint-based nurse rostering system using a redundant modeling approach. Nurse rostering is defined as the process of generating timetables for specifying the work shifts of nurses over a given period of time. This process is difficult because the human roster planner has to ensure that every rostering decision made complies with a mixture of hard hospital rules and soft nurse preference rules. Moreover, some nurse shift pre-assignments often break the regularity of wanted (or unwanted) shifts and reduce the choices for other unfilled slots. Soft constraints amount to disjunction, which can be modeled as choices in the search space. This approach, although straightforward, incurs overhead in the search of solution. To reduce search time, we propose redundant modeling, an effective way to increase constraint propagation through cooperations among different models for the same problem. Our problem domain involves around 25 to 28 nurses and 11 shift types. Experiments and pilot testing of the system confirm the effectiveness and efficiency of our method.