If you’ve paid people, and they haven’t delivered, either you don’t agree on what the deliverables were, or those people are criminals. Remember that most professionals sell time, rather than deliverables.
If that’s not the case (you’re not paying those people anything up front)
Then this talk is a good place to start and the gentleman’s book is called You’re my Favorite Client and is all about hiring and working with designers specifically but really applies to web developers almost as well. It would give you some norms and tools to evaluate people for the project.
please watch past the title on that talk, I’m not trying to insult you, the guy is just direct and names like “f*kc you, pay me” fill the seats
Something is wrong if you’ve gone through four developers. I’m not saying it’s you, them, the project, the company, the payment arrangements, or the work, but it’s at least one of those. Web developers are just like plumbers or roofers or mechanics, they want work and they want to get paid, so turning down work and upsetting clients isn’t the norm and isn’t good for anyone.
I’m not sure what sorts of research and talking points you’re looking for, and all the previous answers are moderated so these may be the same points they’ve made.
If you’re looking for something that says “47% of web developers don’t finish the projects they start”... that won’t be out there. It’s not that different than hiring anyone else.
But if you want a bigger pool, you can use that to your advantage and find talking points related to more general home improvement contractors. A quick google search turned up several government guides to dealing with them and how to protect yourself from common deceptive practices.