Again, the method isnt exactly the same. The study I linked to studied individuals states and even counties in which the executions were performed against areas in which the death penalty was not enforced or didnt exist. Again, the method used wasnt the same method argued against.
I'll again recommend that, after you retire, you enroll in a complete statistics course (like one offered in a university). You know what I mean, the 30 hours of lecture/45 hours of study kinda thing. One with a professor and a TA to work you through specific examples.
Maybe a course on research methods and design, though I don't think that's nearly as likely as it would require more than one semester.
You LOVE these types of arguments, a great deal. But "Common Sense" does not equip one to properly critique/examine research studies.
That said, I do agree that lawyers should be using science to streamline the legal process. Proper datamining technologies would be a godsend in this field. Speech-to-text would reduce the stress of dictation burdens on the assistant staff. More rigorous science would allow evidence collection to be more reliable. Etc.
I'd think, though I'll not argue heavily, that increasing the chance of a (proper) conviction for a serious crime would have the greatest deterrence effect. The greatest benefit would be realised if we could get young men to mature out of the 'aggressive hormones' stage of young-adulthood!
I also agree that a society could argue that a system which resulted in some innocents dying but saved a LOT of oblique suffering could be defended. They have to be careful if they're killing their own citizens, because the 'societal contract' is sticky at that level