This could help you.

http://www.ncbi.nlm.nih.gov/pubmed/16683464

Actually this paper gives more of demographic statistics but the question of addressing the manner remains unanswered.

Manner is a difficult question. It needs all collation of all circumstantial evidence, along with the postmortem examination findings (thus scene, eye witness and participants in the case's evidence, etc. all play a key role). An even bigger contributor in determining the manner is if the victim was able to make a dying declaration about her circumstances of death.

I would think the one situation where all things come together in a decent system of death investigation and death investigators is the US Medical Examiners' system, but I have qualms about their interpretation of the manner of death automatically transcribed on all death certifications - where the ME's manner may be at variance with what the courts find out later!