Answer: According to the William Estes's stimulus sampling theory, each stimulus (in this case, faces) are made up of a variety of elements. As these faces are learned, part of these elements are randomly sampled on each trail, and only sampled elements can be associated with the response. Sometimes, it may occur that, on a particular trial, a subset of elements (that are not strongly linked to the correct response) is activated. Consequently, it leads to an error.
