Arthur Miller wrote a “Note on the Historical Accuracy of this Play” at the beginning of the Viking Critical Library edition:
This play is not history in the sense in which the word is used by the academic historian. Dramatic purposes have sometimes required many characters to be fused into one; the number of girls involved in the ‘crying out’ has been reduced; Abigail’s age has been raised; while there were several judges of almost equal authority, I have symbolized them all in Hathorne and Danforth. However, I believe that the reader will discover here the essential nature of one of the strangest and most awful chapters in human history. The fate of each character is exactly that of his historical model, and there is no one in the drama who did not play a similar – and in some cases exactly the same – role in history.
As for the characters of the persons, little is known about most of them except what may be surmised from a few letters, the trial record, certain broadsides written at the time, and references to their conduct in sources of varying reliability. They may therefore be taken as creations of my own, drawn to the best of my ability in conformity with their known behavior, except as indicated in the commentary I have written for this text. (p. 2)
Read more here.