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The mechanics of Call of Cthulhu

It was cold out so I spent my lunchtime developing a systems dynamic model of the SANity mechanism for Call of Cthulhu in iThink.

It's not massively sophisticated at the moment. There's a reservoir of SAN that depletes at random moments with a SAN roll (either 1 or d20) and there are random top ups of SAN (called End_of_Adventure) that have the opposite effect (d10). I've added in Cthulhu Mythos which randomly increases by d3 every so often and limits max SAN.

The graphical interface just has a knob for setting starting SAN, a button marked "Run" and a graph of SAN against time with lines for SAN loses and gains (see cut for piccy).

It's nothing terribly flash as far as iThink is concerned. After all, the software was built for running simulations of assembly lines and complicated dynamical processes such as crisis management of epidemics. So a bit of roleplaying? No sweat.

On the other hand, it has got me thinking about the fact that I should really write some of this down in a more structured fashion. I know someone's done a big paper on the mechanics of various games (I'll find a link later) but it's really just taxonomy. It doesn't talk about the various kinds of structures that you get in these systems from the basic negative and positive feedback loops to the more complicated causal loop diagrams (not casual loops, those are much more laid back and would rather spend their time on the couch with a beer discussing the phenomenology of socks than doing anything else), nor does it show you how to build systems that produce the kind of design features that you'd like in your game.

The fact that this is what I'm paid to do during the week might explain my reluctance to do it a weekends but it's a nice idea.

If "causal loop diagram" makes your brain hurt, you could try the MIT introduction here


( 3 comments — Leave a comment )
Mar. 14th, 2006 02:09 pm (UTC)
I think I just lost san trying to understand all that.

Am I right in thinking the graph suggests a lifespan of 35 months of adventuring before reaching zero san? Sounds a bit optmistic to me :-).
Mar. 14th, 2006 04:32 pm (UTC)
This is not an average, this is just one run. There's plenty of variability between different runs and I'll have to do lots to get an average time to looniness.

The time axis is purely arbitrary. I left it at months because the frequency of rolls depends very much on the type of game. There's basically a SAN roll every "month" leading to the loss of either 1 or d20 SAN.

And there's a SAN gain of 1d10 every 1d6 months representing the gain at the end of an adventure.

Running the model 100 times with a starting SAN of 70, only 4 PCs hadn't gone mad after 40 "months", the majority of them succumbed after 20 "months".

I suppose to better follow adventure design SAN rolls should start off small 1/1d3 and progress onto a big one at the end typically 1d10/1d100. And eventual SAN gain should be higher the bigger the potential loss at the end is.

It's a prototype. Eventually, you won't need to play at all. Just press "run" and watch your PC go mad.
Mar. 16th, 2006 07:11 pm (UTC)

I look forward to the full results! ;)

( 3 comments — Leave a comment )