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Histogram 2 is one of the stats issues I am stuck on. There are multiple party members, so just registering the individual creatures loses some data. I've chosen some fairly arbitrary "buckets" right now. I could go for %age total hitpoints remaining. Then it's a matter of how I would note dead characters. 0% for a dead character misses an important piece of information. One dead or unconscious character and two at half hitpoints is a very different play experience than 3 characters on one third of their total hitpoints.
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I don't have much problem with the buckets approach to describing health. It is a reasonable approximation to how a player would think about it and the actual numbers are probably less interesting then the descriptive bucket. Plus, you can change your bucket parameters to see if it materially affects your conclusions later.
Regarding the dead/runaway/low/medium/high problem... I think you need to attach a value system to the overall "adventure outcome". We want to be able to say something like "the PCs can continue to adventure X% of the time, require additional resources (healing) Y% of the time, and face a minor or major pause (extended rest, go to town, etc.) in the adventure Z% of the time."
I would propose assigning values to specific dwarf condition outcomes as follows. It is arbitrary, so you could tweak the numbers to better reflect player feelings.
First, assign values to each dwarf using a Condition of Dwarf Function (CDF*):
if Dead then -5
if Runaway then -2
if Low hp then 0
if Medium hp then 2
if High hp then 5
Then, assign values to the generic outcome with a Fun Factor Function:
if ALL SURVIVE then 5
if TPK then -5
Outcome Value = OV = CDF(Dwarf 1) + CDF (Dwarf 2) + CDF(Dwarf 2) + Fun Factor
If everybody comes through unscathed, OV = 5+5+5+5 = 20
If everybody comes out with Low hp, OV = 0+0+0+5 = 5
If two have low hp, one with high hp, OV = 0+0+5+5 = 10
If everybody dies, OV = -5-5-5-5 = -20
etc.
Interpreting these numbers:
1) Anything very positive means the party can keep fighting without much issue.
2) Anything very negative means the adventure is effectively over.
3) Anything slightly positive means the adventure keeps going, but they need additional resources.
4) Anything slightly negative means the adventure pauses, but some event has to occur to continue. Go back to town, new PC, DM deus ex machina, etc.
Interpreting the results:
Slightly positive numbers are the sweet spot. We want the PCs to be challenged by the monsters, but we consistent negative outcomes (of any kind) will slow down the campaign and perhaps aggravate players.
If we see negative numbers 25% of the time, 1 in every 4 sessions will require some minor or major derailment of the adventure.
If we see large positive numbers 25% of the time, 1 in every 4 session will see fights where the PCs are not significantly challenged.
An "easy" game system would have mostly positive numbers. A "hard" game system would be close to zero.
IMPORTANT CAVEAT: My choice of values is arbitrary and I will not defend them. I made the function symmetric, but my experience suggests that "bad outcomes" probably should carry more weight. Following cognitive psychology and prospect theory, I would guess that "death" should be weighted 2.5 times more than "high hp".
* Yes, this is a lame statistics joke.