Shakespeare attribution studies is the scholarly attempt to determine the authorial boundaries of the William Shakespeare canon, the extent of his possible collaborative works, and the identity of his collaborators. The studies, which began in the late 17th century, are based on the axiom that every writer has a unique, measurable style that can be discriminated from that of other writers using techniques of textual criticism originally developed for biblical and classical studies. The studies include the assessment of different types of evidence, generally classified as internal, external, and stylistic, of which all are further categorised as traditional and non-traditional.