Question:
The stats I've seen just say "dead" they don't say why dead.

Meaning they are not controlled for death of head injury vs other kind of dead, nor for type of head injury.

It's quite possible, for example, that open face wearers ride different roads, have different types of crashes, and end up dead because of that, possibly not even from head injury at all.

What studies have you seen that specifically mention the type of head injury that caused death, and what was the sample size?

Answer: The helmet is there to protect your brain rather than your physical appearance. As they all have to pass the Australian Standards test ( I know this has been argued to death here recently ) it is a reasonable assumption to make that they will all protect you from serious or bad head injury - brain trauma - in a survivable accident regardless of whether or not they have a chin protection. But from memory they were somewhere between the open face and full face. I would actually consider getting a flip up style, because ot me, they are less ridig than the full face, but they still prvide protection to teh face. Tho im waiting for one that you can easily change the visors, all the ones ive seen, you ahve to screw and unscrew to change the visor.