The only works of Shakespeare about which we can feel reasonably confident that we have the versions that the author intended are the poems Venus and Adonis and The Rape of Lucrece. That is because, in the early part of his career, Shakespeare appeared to think of himself more of a poet than a playwright and he wrote the latter largely as a means of supporting himself in the role of the former. As a result, it is known that he proofread the published poems before publication. The same is not true of the plays. Playwrights in Shakespeare’s era made very little money from the publication of their plays, partly because no more than one thousand copies of the book were likely to be printed and sold and partly because plays were only published when audiences were no longer really interested in watching them being performed. Consequently, there was little incentive for dramatists to worry too much about the final text and, by and large, they did not.
A second reason for the poor quality of published works is that they were often improvised during performance and written and rewritten during production to make them more effective on the stage. Leading actors, for whom some of the biggest dramatic roles were written, could have lines changed for them or even create their own. In addition, many plays were not the result of one author but of collaborations between two or sometimes half a dozen. In order to create new plays to order for the stage, it was sometimes necessary for playwrights to work together, perhaps specializing in areas of their greatest strength. It is common these days for television and feature films to have teams of writers and for some of the more experienced writers would adjust some of the initial scripts during production if for any reason they are felt to be inadequate or inappropriate. The same was surely true in the Elizabethan age.
The result of this is that there are a number of works which Shakespeare is supposed to be part author but it is not entirely certain. Of course, there are many scholars who believe that one or more of the plays traditionally accorded to Shakespeare were written by someone else. Much of the debate over the years has been unscientific but recently computer-aided textual analysis has been used to try to establish the authorship of problematic plays by considering the particular words and phrases associated with particular authors. Currently, this remains somewhat controversial but it offers an additional and interesting way to try to establish Shakespeare’s real list of achievements.