A method performed by a computer (200), the method includes obtaining a plurality of images each of which includes a plurality of animals (5a,5b,5c), the plurality of animals being attached with a plurality of attachment objects (6a,6b,6c) respectively, the attachment objects having respective different colors, using at least one image among the plurality of images, detecting the colors of the plurality of attachment objects respectively, using the plurality of images, detecting movements of the plurality of respective attachment objects, based on the colors of the plurality of attachment objects, and in accordance with the respective movements of the plurality of attachment objects, determining respective activity quantities of the plurality of animals.
