Friday, 25 January 2013

WoD percentifier


A work in progress: discussion on the Dr&Di thread.

This is a fettle in response to Arthur's modest proposal on Dreamers and Dicepools.

This script will (I hope) convert a World of Darkness-style dots and targets system into a percentile chance, allowing a quick and dirty conversion into BRP or some similar system. You can use this to determine what skill percentage corresponds to a dot rank.

Your percentage chance of success is:



But that's not the only option. You may be keen to import your old Call of Cthulhu characters into the World of Darkness, and who am I to stop you? This called for actually quite a lot of effort.

You need this number of dots:


Note: At some point in the rather complicated process of moving things between HTML editor, spreadsheet, plain text editor and back again repeatedly multiple times, the exact numbers to many decimal places have apparently got slightly broken. So this will in fact mostly return very slightly inaccurate numbers, until I get round to fixing that.

So it looks like it's actually probably a Javascript issue with logarithms, not an error in the data. This might take longer than I hoped. Or I might come up with a nasty hack for it. Nasty hack found, and not actually all that nasty. Javascript is actually not that good at maths, it seems...

Nerdy calculationy bit

The Percentifier is a rough and ready option, right there. But it would be nice to have a way to convert the other way, right? To be able to convert a mathematically-elegant BRP character, with its straightforward capabilities, into WoD’s dicepools.

This, unfortunately, is going to call for maths. Wait, I don’t dislike maths. I just don’t have to do very much of it these days.

The calculation for turning the dicepool into percentages basically points down to:

p = 1 - ((t-1)/10) d

where p is the probability of success, t is the target number, and d is the number of dots (or dice).

At this point I realised I couldn’t remember high-school maths. Things like “what’s the general term for the power of something, like ‘squared’?” which would help answer other things, like “how do you calculate the value of that thing I can’t remember the term for?”. Luckily, at this stage my dad called for a chat. Amongst his many excellent qualities, he is professionally good at maths, and reminded me about logarithms and the word ‘exponential’. Thus:

p + ((t-1)/10) d = 1

1 – p = ((t-1)/10) d

LOG(1-p) = LOG(((t-1)/10) d)

ln(((t-1)/10) d) = LOG(1-p)

dLOG((t-1)/10) = LOG(1-p)

d = LOG(1-p)/LOG((t-1)/10)


But there's more! Arthur has a suggestion for making skill more relevant in WoD, and suggested how it might work, so I've fettled up a quick Arthurfier (for want of a better word) to see how that does.

EDIT: This doesn't, in fact, do what Arthur suggested: it just triples the relative value of Skill vs. Stat dogs dots. It's been a long few weeks, okay?

Your percentage chance of success is:


Arthurfier II

Thanks to me rolling a critical fail on my Reading, the Arthurfier doesn't actually do what Arthur suggested. Oops.

Enter the Mark II.

Your competence at this task is:


Your percentage chance of success is:



I eventually managed to track down the descriptions from WoD.


  • * Poor. Unexercised, unpracticed or inept.
  • ** Average. The result of occasional effort or application.
  • *** Good. Regular practice or effort, or naturally talented.
  • **** Exceptional. Frequently applied, tested and honed, or naturally gifted.
  • ***** Outstanding. The peak of normal human capacity. Continuously exercised or naturally blessed.


  • * Novice. Basic knowledge and/or techniques.
  • ** Practitioner. Solid working knowledge and/or techniques.
  • *** Professional. Broad, detailed knowledge and/or techniques.
  • **** Expert. Exceptional depth of knowledge and/or techniques.
  • ***** Master. Unsurpassed knowledge and/or techniques. A leader in the field.

I can definitely see why Arthur was unconvinced.


  1. How does the Arthurfier actually work? I fiddled around with the dots and can't seem to make it fit my proposal. (For instance, I select 5 skill dots and 3 stat dots and a system where 1 skill dot counts for 3 stat dots, which under my proposed system would mean you have 18 dots total translating to a 90% base chance, but none of the target numbers give 90% as the probability).

    1. FAIL. I read the first part of your suggestion, and apparently skipped the second bit. So the first-gen Arthurfier actually just uses basic WoD but with skills multiplied by three.

      Fear not; we can rebuild her!

    2. That's something I could roll with. :D