In my previous post, I mentioned that in my weekly campaign we average around one PC death a month. In the comments somebody asked me about the causes of these deaths, and that sent me down a statistical rabbit hole which probably reveals very little (it's a tiny sample; there are lots of different operant variables in any given case; a lot of it is down to chance dice rolls) but which interested me nonetheless.
The list of PCs killed, and causes of death, are as follows, where the cause of death is one of three categories: random encounter, killed by keyed dungeon or wilderness denizens, killed by trap, natural disaster or other environmental factor, and killed by set-piece 'boss fight'. I don't have a 'plot arc' in my campaigns, but in my dungeons and hexmaps I do tend to have a number of 'bosses', and there are also occasions in which the PCs make powerful enemies and their entanglements take on a climactic character. (Also, there are two PCs for whom I have simply forgotten the cause of death.
Gnaeus, 4th level Roman Cleric, killed by a spider-elf (boss fight)
Xanthippe, 3rd level Roman Fighter, killed by crow-men (boss fight)
Aurelia, 3rd level Celtic Fighter, killed while escaping from the Swan Queen's prison (boss fight)
Amyntas, 1st level Macedonian Fighter, killed by earwigmen (keyed dungeon denizen)
Men-Kheper-Ra, 2nd level Egyptian Magic-User, killed by tree guardians (random encounter)
Kemnebi, 5th level Egyptian Thief, killed by woodwoses (keyed dungeon denizen)
David of the Web, 1st level Celtic Fighter, killed by crow-men (boss fight)
Finan of the Hammer, 6th level Celtic Fighter, killed by insect-elves (keyed dungeon denizen)
Argyros, 1st level Greek Cleric, killed by lacewing automata (keyed dungeon denizen)
Bomilcar, 1st level Carthaginian Fighter, killed by shrew-men (keyed dungeon denizens)
Pry, 1st level Celtic Fighter, killed by tree guardians (random encounter)
Stymatos, 1st level Greek Magic-User, killed by insect-elves (boss fight)
Octavius, 3rd level Roman Cleric, killed by [?]
Laren Dar, 3rd level Etruscan Fighter, killed by a spider-elf (boss fight)
Pupli Artnli, 2nd level Etruscan Cleric, killed by earwigmen (random encounter)
Wolvela, 2nd level Celtic Fighter, killed by [?]
Atrius, 3rd level Roman Cleric, killed by a giant spider (random encounter)
Flavius, 4th level Woodwose, killed by a cyclops (random encounter)
Cutheyura, 1st level Atlantean Fighter, killed by a giant swan (boss fight)
Pandion, 5th level Greek Magic-User, killed by tree guardians (random encounter)
Mastanabal, 1st level Carthaginian Cleric, killed by wererats (keyed wilderness denizen)
Padraig, 7th level Celtic Fighter, killed by a giant scorpion (random encounter)
Flewyn, 7th level Celtic Thief, killed by giant ants (boss fight)
Plotted on a pie chart, this shows us:
This suggests roughly even amounts for all categories except traps and other environmental factors. This may simply be a result of taste; I don't really like 'gotcha' traps, magic ones in particular, and prefer them to be at least reasonably realistic. This means that intelligent players will generally spot them and figure them out - they work more as bottlenecks or roadblocks than life threatening dangers.
Otherwise, the really noticeable thing is I suppose that almost a third of deaths occur in random encounters, indicating that they are very much to be avoided; not only are they dangerous, but PC deaths in that context will be meaningless and usually will be for little reward.