By 1939, in his Prolegomena for the Oxford Shakespeare, McKerrow had changed his mind about this approach, as he feared that a later edition – even if it contained authorial corrections – would "deviate more widely than the earliest print from the author's original manuscript." He therefore concluded that the correct procedure would be "produced by using the earliest "good" print as copy-text and inserting into it, from the first edition which contains them, such corrections as appear to us to be derived from the author." But, fearing the arbitrary exercise of editorial judgment, McKerrow stated that, having concluded that a later edition had substantive revisions attributable to the author, "we must accept all the alterations of that edition, saving any which seem obvious blunders or misprints."

Why did McKerrow alter his previous method of criticism?