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To increase the validity of evaluations and facilitate expansion and maintenance of assessment systems, we constructed a database of studies on the welfare of laying hens around the world. On the basis of this database, we devised a science-based welfare assessment model. Our model includes measurements, levels and weightings based on the scientific studies in the database, and can clarify the advantages and disadvantages of housing systems for laying hens from the viewpoint of the five freedoms. We also evaluated the usefulness of our model by comparing it with environment-based Animal Needs Index (ANI), another science-based model called FOWEL, and animal-based measurements. Our model showed that freedom from injury, pain and disease, and freedom from discomfort were more secure in the cage system, while non-cage systems scored better for natural behavior and freedom from fear and distress. A significant strong-positive correlation was found between the animal-based assessment and the total scores of ANI ( rs = 0.94, P < 0.05), FOWEL ( rs = 0.99, P < 0.05) or our model ( rs = 0.99, P < 0.05), which indicate that these different approaches to welfare assessment may be used almost interchangeably to 'measure' a common property ('overall laying hen welfare'). However, assessments using our model and FOWEL were more sensitive than ANI and can be applied to cage systems, which suggest that our model and FOWEL may have added value. [ABSTRACT FROM AUTHOR] more...