The murder of Elizabeth Stride has long been considered the odd one out of the 'canonical five' murders. Down the years she has been ruled in and out by numerous Ripper authors and amateur sleuths but the consensus seems to be that she was probably a victim of the serial killer known as 'Jack the Ripper'.
The question is why?
Stride's throat was cut less than an hour before a Ripper-esque murder. Sure, the timing is convenient, but life is full of strange coincidences. The Ripper case certainly isn't an exception to happenstance, e.g. Eddowes giving the same name to the police as the next victim, and Sarah Brown having her throat cut in a domestic on the night of the Double Event.
The lack of mutilations. These are largely attributed to an interruption, hence the savagery of Eddowes' attack by an assumed frustrated Ripper. This is purely speculative. If mutilation was the key, why would the killer attack his victim next to a busy social club? Furthermore, the eyewitness description given by Schwartz of Stride's assailant doesn't jive with the previous murders. And if this man wasn't Stride's killer, it only leaves a small window of opportunity for another murderer to enter stage left.
All we have are two women that are murdered within an hour of each other. Both of them have their throats cut but aside than that there is no evidence that the two murders are related. The only evidence that links them is a letter written by someone claiming to be the killer, which is mainly considered a hoax by most authorities on the subject.